AI teams can hold up to 6 bots, but by default they're only programmed to hold 3. There's nothing magic that you need to do to go from 3 to 6, the devs just decided to stop at 3 per team for some reason (assuming you're talking about the stock AI). If you want to know how to add bots to the AI teams, you should look up the tutorials section for tutorials about 'how to AI'.
To be more specific, the line underlined below tells RA2 which bots to look for in a team folder. In the example below, it's telling ra2 to look for Bot0.bot through to Bot5.bot. In stock, the line would be 0 1 2 instead, telling ra2 to look for Bot0.bot to Bot2.bot, thus only having 3 bots in a team.
