I see this more and more, the server shows 12 to 16 people in the game, yet it gives me this message quite a bit. Yet other times I have connected just fine when it was at 18/22 first try. It happens when loading from the Steam desktop, it does this when opening from within CS:S game itself.
Like right now it varies from 12/22 to 14/22, 3 admins (which should be using the reserve slots) and it will still not let me in the game with this same error message. I have been trying to connect every minute or two for the past half hour with the same error. Earlier when I was in the game I saw others connecting every minute or two but not able to join the game either.
Please look into this, I do not think the admins are taking up the reserve slots like they are supposed to be (currently has Cry Wolf, Smurf and U Suck with the ibis.a tags).
I have been a server admin and owner (other servers) for many years myself so I know this is a server problem, not my client or game.