JoeJ said:
If unit number X is the target, they will scale detail to whatever remains possible. Number of units / NPCs is harder to scale than graphics, so that's surely the more important limit than polygon count. But having no RTS experience i can only guess myself.
So, in terms of RTS (now I'm hardly an expert - but I did internal attempts on both RTS and MOBA as proof of concepts, apart from interesting demos one can hardly call it a complete project - it also wasn't in demonstrable state).
As long as we stayed in realm of single-player nothing really was a problem on hardware since like 2006 or so. The big problem started appearing elsewhere, and I'm sure you know where - it is in multiplayer. Unless you want 1-on-1 you should stick with client-server architecture (and even for 1-on-1, cheating or such will be a huge problem, without server in place it is very hard, sometimes even impossible to detect such), and that is where problems begin…
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Now, if you want to have 3000 active units in the game, if each of them is individual just keeping units in sync at 30 ticks per second will need quite a lot of data (position, orientation, velocity, angular velocity and some additional state variables, etc.), let's say we fit that all in 48 bytes of data. This still means you need 3000 * 48 * 30 = 4320000 bytes per second bandwidth (over 4 MB/s transfer rate). While one may object that we don't need to transfer inactive objects, keep in mind that this is RTS - it is more than likely that ALL of the units are actively moving, fighting or doing something at once. You will also need to transfer other, more static objects (buildings and their state changes, resources, etc. - but those are significantly smaller in terms of bandwidth - also those are somewhat easier to optimize - unlike dynamic units), but we can ignore them for now.
Sure, you can decrease ticks. Sure you can compress data based on some assumptions (to some extent). Etc. … So yes, you can optimize - but for each optimization you will need to sacrifice something (being less generic, additional conditions, limited map size, etc.). Each of the optimizations will be a trade-off.
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Now let's say you optimize this down to fit in 512 kB/s requirements, optimize things to reasonable level … keep in mind that you're still running on a server. That also means you (or the one hosting games) have to be running the server.
How many server-side game instances are you able to run on single server (you will be limited by performance of the server and total bandwidth on the server)? Having both high means high running costs.
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Most of the multiplayer focused real time strategies do limit actors in some way, examples:
Warcraft 3 - Introduced upkeep, limiting “food” to limit number of actors in game
Starcraft 2 - Limited supply for units
Age of Empires 2 - Max. supply per each player in game
Spellforce 3 - Limited supply for units
And I could go on.
