As a mission designer you, of course, have the creative freedom to craft any scenario you desire. I will warn you that pushing the limits REALLY can lag player's experience especially when you get up over 25 players.

I personally hope most designers will design maps for 30-40 players since our server is that popular currently, particularly at weekend events. With this high of count, its not wise to push the limits as it will lag the server.

Now there is one thing to this. If your going to make a huge map (hundreds of enemies, vehicles, 30-40 player etc.) To avoid lag you can spread the enemy, action, and players out across the entire map. ArmA 2's game engine will get bogged down if all the players are right with each other in a group and engaging in heavy combat at higher player counts. So, spread things out. Also, keep the view distance lower. (say 900m max) because A.I. simulation cycles are directly related to the view distance. The larger the view draw SERVER SIDE, the larger the load for AI behavior calculation is since it is calculating out further. You can always include a script for individual players to adjust their view distance on their end (keep an eye on forums, I got one I made and I'm releasing it today).

Maybe make several teams with different objectives, and maybe some players off somewhere who can provide air support. Spread the action and objectives all over the map to keep distance between them. All of this being part of a larger campaign on the entire map. Perhaps you could place one unit to act as the mission commander. He sits at a base, with all the objectives on his task list. So he can make sure the entire theater of operations gets done. Remember with this new commander interface in the game you can have multiple commanders who are each over seeing multiple groups.

Just some food for thought. Hope that helps.