I don't have the team or time to make a game to the scale of LoL and DOTA. Most likely I would be sticking with 1v1 and 2v2, but I wanted to see if it would be feasible to push the limits to 3v3.
Heaven knows 5v5 would be a nightmare.
Regardless if it is 2v2 or 10v10, number of the player is not the limitation, what can limits you is your server machine, can it handle bandwidth for several instance of 5v5? Will it have enough processing power to process several instance of 5v5?
In C2 MP context, this is largely depends on how you build your event, there's no clear cut on how to optimize the bandwidth and MP game itself, it's a matter of preference and choices whether you rather sacrifice bandwidth for smooth "lively" movement or having a lot of complicated send message (used as trigger) to save bandwidth when some event can be internalized in peer and host.
I think it is feasible in scale of 100-200 (might be more) concurrent players or so for one server machine, with good bandwidth.