As mentioned, the size of fleets really determines performance more than anything. I find a good system can usually handle about 8k command before it starts to become unplayable. Unfortunately, with ten players that means only about 800 command in units each. That is easily doable if the game goes on for longer than an hour.
More to the point is how defensible you make the map. If the players are placed far apart so they can't be rushed and have excellent choke points to defend, you will very easily build up to game-stuttering fleet sizes. If the map is more open and prone to bloodshed, this will keep the fleet sizes suppressed for longer and improve performance, even if there is a large number of planets.
That said, I'm of the opinion that a map should have in the range of 5-15 planets per player. If you're going smaller or larger, you should have a good reason for doing so.
