Which game mode are you looking to play with your friends? There are mode scripts for
* invasion, which is like single player campaign, all players cooperative in one faction
* classic, also known as PvPvE or sandbox, each player is free to join any faction, there are bots included, map changes when the map is conquered by one faction, King of the hill -maps not included
* team elimination, round based 2-faction PvP, no bots, 1 small special map
* deathmatch, no bots, 1 faction with friendly fire enabled, TKs count as score, extra outifts available at the armory, .. not quite sure now which maps are there in rotation, I guess most of them are not very well fit for deathmatch due to size, but maybe that's fun to someone

Invasion has bot amount control in gamemode_invasion.php in form of multipliers, $fellow_capacity_factor for friendlies, $enemy_capacity_factor for enemies
Classic needs its bot amount set per map, so you'd need to edit map_rotator_classic.php and change max_soldiers for each map.
If you're not using a gamemode script, or wish to intervene with the script, you can also call "max_soldiers XYZ" in the server console. If you have a gamemode script running, mind that you may need to re-set the max_soldiers setting each time a map changes or a new match starts. max_soldiers refers to maximum capacity in the whole map, roughly, so the number you set is split between factions depending on how many bases they have, with some balancing effects. You can see the effective max_soldiers setting in the console by calling "status".