There is a CVAR in R1, not sure if tjw added it, i think he did, i dont know, we are running both at the same time pretty much on sst.
The variable is g_maxgameclients.
Set it to 16 if you want 16 players to be able to play, then bump up the sv_maxclients up to whatever number you want, only 16 will be able to play at a time, all the others will be forced to spec.
This is how AKKA used to do it.
Private slots are seperate from all this, to the best of my knowledge.