pb_maxbots setting doesn't guarante yet You will have the 10 bots on the server. It's a limit only for how many bots + humans You may have on the server and it's also a limit for the functions pb add / pb fillserver.
So You need to use such settings:
pb_minbots 0
pb_maxbots 10
pb fillserver 100
to fill the server by 10 bots if no body is there. If there are already 2 humans, the fillserver will add only 8 bots, because 2 + 8 is 10 (pb_maxbots). More info and examples read in
docs.