pb_maxbots is a parameter which says how many
bots+humans can play on Your server. It's just the
limit only for pb fillserver command or for pb add commands. If You want to have some bots on Your server -
You have to add them by pb add or pb fillserver commands.
More info about settings You can find
here. There is also some info in podbot.cfg - it will help You to understand the settings.