I checked my configuration files, default.cfg actually had the number or slots for private clients smaller than that of maxclients, however, I (got a new main harddrive and took the opportunity to get rid of a lot of software packages and) installed a new system recently, the way I solved it was, to go to console and type
/exec default.cfg
that helped. I have made the experience that Tremulous is a bit buggy concerning configurations sometimes, by simply reloading the config I obviously solved the problem. It works now.