Good and big MP RTS games?

Starcraft 2

Edit: I might have misread. How many people are supposed to play together/against each other?

Take a look at AirMech.

Then you are looking for a MOBA, not an RTS.
Anyway, WarCraft 3 supports 12, World in Conflict 16. (Good luck running with that many units.)
For even older games, Total Annihilation supports 10, Myth II… er, more than 10.

The Forged Alliance Forever community project for Supreme Commander 1's expansion allows 6v6 matches.

supreme commander 1-2.
2 is faster and less complex.

The best answer I can think of is Shattered Galaxy.

It's a persistent online MMORTS with up to 40 players per battle, fighting for control of the map where territory is won or lost depending on the outcome of each skirmish. Technically, the servers are still running and anyone can create a new F2P account; even if in almost every other respect the game is archaic and nearly dead. I wouldn't really recommend it otherwise, but there aren't a whole lot of games that support RTS battles on that sort of scale.

OpenRA is a compilation of old Westood strategy games of Red Alert, Tiberian Dawn and Dune 2000, refined, rebalanced, renewed and can be played globably with friends and strangers alike = http://www.openra.net/

CNCNet is a service created for all old classic Westwood strategy games for them to be played in global online multiplayer with ease, from the very first Tiberian Dawn, to the relatively new fan made games for the series entries like that of Twisted Insurrection, they are not changed unlike in OpenRA, still the same, good old games from the old days and you can easily monitor the playerbase which exists = https://cncnet.org/ this website might also serve you of perhaps some use like game downloads = http://cnc-comm.com/

Starcraft 1 Battle Net is a realiable, awesome multiplayer allowing you to play against a big world of players that have not moved onto Starcraft 2, i dont remember how i set it up for that but it was worth it when i did and i can assure you it will be worth it for you too, many players, many maps and modes to play on. As for Starcraft 2, also a option, cant afford it? Play the limited free version, its decent for the simple matches but for me SC1 is better, 30 out of 10, would get owned as weak terrans by overpowered prottuses and zergs some more.

Age of Empires 3 Complete Edition + Game Ranger = from 800 to 2000 players to play with and against daily as ive heard, better than Age of Empires 2 and has plenty of players, what are you waiting for? Or you could always ofcourse watch this guy's gameplay videos, he plays the game quite often and many other RTS'es = https://www.youtube.com/watch?v=WIjP0fY6FG0

8 Bit Armies is new, is simple and has players.

Act of Aggresion had a big update and surely has players to challange now

Total Annihilation most likely still has players to play against right now still as its a great game and was recently added on steam.

Warcraft 2 has a still functional ofifcal working Battle Net and is a great game to play with players online.

Warcraft 3 Reign of Chaos and Frozen Throne might be quite tricky to set up for multiplayer, but if you can locate the most populated servers and services, youre surely bound to find players still playing all sorts of things, not just ordinary matches and is overall a great game.

Cossacks: Back To War + Game Ranger and you got yourself a awesome game to play against other players, playing the campaigns are also worth it, would help get ready for Cossacks 3 when it released for which you can bet will have many, many players online.

Armorgames.com has a strategy game called Colony that you can play with a partner, against 2 other players, a very classic great game, sometimes you can find some people still playing it but it can be hassle to get things started and even going.

C&C 3 Tiberium Wars, C&C Generals Zero Hour, Red Alert 3 also have players still playing them alot, you could try and hook up with this youtuber called irwe3 who plays with people often and see if you can get into the action = https://www.youtube.com/watch?v=oe0k-2nBa_A

Company of Heroes 2 is always alive, always players to fight.

Total War Arena is a awesome massive army, 10 vs 10 player strategy game to try out aswell.

Upcoming strategy games to keep an eye out for are Warcraft:Armies of Azeroth, Call to Arms, Halo:Wars 2 and Blitzkrieg 3 as you can see here https://www.youtube.com/watch?v=XehVIfKPl-c

Thats all i can vouch for mostly, any other strategy game may or may not have people still playing them like im not sure about Warhammer 40k Dawn of War I and II.

Hope it helps.