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January 22, 2021

Supply & limits

Supply & Limits
Supply & limits

Various limits on the number of units are remarkably common in RTS games, we can find them in games like StarCraft, WarCraft, or Age of Empires. I want to look at various types of these limits and what their effects and functions are.

Supply mechanic vs unit cap

First let's make a distinction for this post. Supply mechanic is the need to build or purchase something to increase the current population limit (e.g. the need to build Pylons). On the other side, unit cap is the maximum limit to which the population size can be increased (e.g. 200 limit). These two things are connected but also distinct, and have different roles.

unit cap

"Unit" or "population" cap is very common. It's a hard limit on the maximum population. It can serve in several functions:

  • It limits how far ahead can one player get. In this way it serves as anti-snowball mechanic, and encourages the winning player to be more aggressive to not lose the advantage.
  • Reaching the unit cap changes the balance between units:
    • This can be used to balance units. For example Roaches in StarCraft II have high supply cost, which makes them a lot more useful early, but encourages players to transition out of them. Supply cost is a parameter that's particularly important in later stages of the game.
    • Different unit compositions are made viable in different stages of the game because of this. Early you might prefer better value per resource cost but later per supply cost.

  • Reaching the unit cap also provides a meaningful choice between the economy size and army size. Additionally, it can make some resource gathering strategies more optimal similarly as with army compositions.
  • There are technical reasons – there can be only so many units before any game engine will crumble.
  • And stylistic reasons – for example Warcraft III is focused on heroes leading a small group of supporting units. A unit cap is the simplest way to prevent players from breaking this fantasy.
The fantasy of Warcraft III is a small army led by heroes

Some games will do without supply. For example C&C3 is fine without it. Hard counters, strong support powers, aggressive gameplay and limited economy keep players from amassing huge armies.

Typically RTS games with more exponential economic growth have some kind of cap. Even Supreme Commander has one, although it's high as the game is made for huge armies.

limit for specific Unit types

This is a special case where certain unit types have their maximum number limited regardless of how big the army is. Examples of such limit would be any heroes (Dawn of War, Warcraft III), epic units (C&C3) or other units (Mothership or Brutalisks in StarCraft II).

For heroes, epic or experimental units this makes sense, but for others it might feel inelegant, especially if the limit is higher than one. The limit provides freedom for developers to make this unit type strong without causing players to amass just this unit. However, if the unit is that good, it's less of a choice to build it or not.

Overall, such limits can be helpful, but I believe we should always try to do without them. Nova in StarCraft II has instead a soft cap due to high cooldown on spawning each unit type. This soft cap can be stretched with certain commander synergies (Karax) or effectively broken with her prestige variant. Non-stackable effects and well-defined unit niches can also lead to something like a soft cap. A Zerg player in Brood War doesn't want just Defilers in the army.

Supply mechanic

Games have their own ways to increase the current population limit. A few common ones are:

  • Supply provided primarily by non-production structures and units (Age of Empires, StarCraft, Warcraft)
  • Mainly by production structures (Iron Harvest, Immortal)
  • By upgrades (Ashes of the Singularity)

Role of the supply mechanic in games:

  • Slows army boom in all stages of the game.
  • Anti-snowball effect – lets a player rebuild easier after taking losses compared to adding more units to an already standing army.
  • Unit and structures can serve as a target for harassment that only slows production down and costs a bit.
  • Supply structures give a stronger impression of building a base.
  • Often serves as a mechanical skill check that rewards good multitasking with smooth production.
  • Simple periodic actions like this can feel satisfying and lead to the flow state.

The anti-snowball effect is particularly interesting. I want to list a few other mechanics that make rebuilding units easier than building more units on top of an army:

  • Squad retreat & reinforce (~50% cost in Company of Heroes 2)
  • Unit repair (25% cost in StarCraft II)
  • Ammo/energy/fuel cost to maintain units (energy consumed by firing in Supreme Commander, munition in Company of Heroes for abilities)
  • Upkeep (Warcraft III, Company of Heroes) – fewer units mean higher income
  • Killed units provide some bonus on death that feeds back to production (e.g. Stetmann's/Horners's scrap, Mengsk's weapons, and Fenix's AI Champions in StarCraft II)
  • Scrap/wreck mechanic is similar but with a good chance of the enemy getting the bonus (C&C3, Supreme Commander, Company of Heroes)
  • Direct cost refund when a unit is lost (World in Conflict)
  • Tug-of-war games can be seen as an extreme case where units cost nothing but time to rebuild, and only production structures cost resources.

★ ★ ★

Power plants in C&C3

Somewhat similar mechanic to supply is power in Command & Conquer games. Instead of units it affects structures. Few of its functions from my article about C&C3:

  • Off-loading some cost to power plants makes replacing static defense cheaper, and make big tech switches easier (you already have power). That supports the core gameplay well.
  • Power plants are often a target of harassment to cause power outage or force building new ones.
  • Power management adds depth to build orders and provides some micro potential when manually unpowering buildings.

★ ★ ★

Is there a way to make the supply mechanic more interesting than just periodically building houses? Attaching more utility to structures or units is one way. Good examples can be found in StarCraft II where Overlords will increase supply but also can scout, transport units, spread creep or morph into Overseers to provide detection. Orc Burrows in Warcraft III can serve as bunkers, and Night Elf's Moon Wells will refill mana to casters.

One issue is that casual players might spend a disproportionate amount of time on "building houses" without enjoying it. There is nothing worse than to watch a casual player building twenty Pylons without using the shift-key or hotkeys. An experienced player might spend a few seconds on it, but this casual player was spending minutes on it each game. That can't be a pleasant experience.

I believe that Age of Empires gets a pass on this, as building your base is inherently enjoyable in that game for all players. However, it's different for a combat focused game like StarCraft.

One option is of course to make fewer of these buildings necessary (~6 in WarCraft III compared to ~20 in StarCraft II). That can alleviate this significantly. Tying it to production structures is another way, or creating not-quite optimal AI or mechanic that will build supply structures automatically but leave space for more dedicated players to optimize and multitask.

Ashes of the Singularity has an interesting approach that ties supply increase to upgrades. These cost a special resource (Quanta) that is also used for research and support powers. This provides a meaningful choice between army size, research and support powers.


Both supply mechanic and unit cap have their place in RTS design. I hope I made some of their roles a bit more clear, and showed examples how they are handled. Here are few links to check out:

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