I would have to say that 6 bots (3 vs 3) is fine.
However, changing it to 4 bots (2 vs 2) would be better because if people start to join in, the teams will be more fair comparing tot 6 bots.
When bots are limited to 4, teams will always be fair: the worst situation is 2 humans vs. 1 human + bot
When bots are limited to 6, teams will be most likely unfair: the worst situation is 3 humans vs. 1 human + 2 bots
We all know that bots are useless most of the time
However in both situations, teams will end up unbalanced eventually
When 4 bots: 3 humans vs. 1 human
When 6 bots: 4 humans vs. 1 human + bot
This is possible because when a human joins a team, a bot dissapears.
I'm not sure if this has been prevented but I know this was the case in the past
Anywho, I voted 6 bots
because when the time arrives for possible screw-up in teams (explained above) the server is most likely filled with players that care for even teams ( there is also a higher chance for even teams).
Including more bots is not recommended.
B2 is not a bot server but only uses bots to keep players busy when server slowly fills up