Finally got around to testing some entities and the reset method that Solokiller mentioned works perfectly (func_door tied to a game_counter_set to reset the game_counter entities each round). I tested it out just by tying the counter to a light and just killed myself for a few rounds to make sure it would repeat.
Being able to reset the counters this way each round is one cleared hurdle. The other now is how to set up the "counting mechanism".
This might be easy depending on how a map is set up and why the players are being counted, but for the specific thing I'm doing it's gonna be a bit more complicated. Not impossible, but hard to make "look clean" to players.
Basically, what I'm doing is creating a "demolition style" map. A small map with one bombsite like the ones in demolition mode in CSGO. But there's more to that game mode than just small maps. Obviously, recreating the gungame-esque weapon system would be impossible to do without actually making a mod, so I just want to capture some of the spirit of the game, and do do this, I want to lay limited weapon pickups to in effect create "classes" (since typically everyone is at different weapon levels in demo in csgo). Despite the map being designed for smaller player numbers (max of 10 players would be ideal) I would need a contingency in place for the server I play on and other servers with more player slots than that. Can't have a map without enough spawn points in the event more people join up. So I need the map to accommodate up to 30 players (even though it won't play that well) and it gets complicated if you are trying to limit weapon pickups. So basically, the player counters will be in place to open some extra doors with more weapon pickups behind them when more players are on while keeping them sealed off when there actually is a small amount of people on.
The easiest thing would be to drop players from spawn through some trigger multiples and into some water. That way there would be no risk of some random lost player going back into spawn or some afk-er tripping a trigger multiple once it becomes ready to fire again. But this would look really crappy to players. The best way would probably to do that same drop method but in a dark room with a teleport right after the trigger that plops them into the "real" spawn. I'm just worried about having too many entitiy setups and crashing servers. I recommended
de_mesa to be added to the rotation on my clan's server a while ago and it would crash the server. However, there are a lot of fancy things going on on that map (which is a shame because I really wanted to see how it played).
EDIT: I forgot I already explained what I wanted to do with my specific map in the first post but, oh well, I explained it in slightly expanded fashion.