Fill the open positions with AI players
Ashes of the Singularity II allows human players and AI bots in the same online match. In the player list, use the available AI choice for any open position you want a bot to occupy. Check the team assignment for each bot before starting, especially if you want all humans on one team.
Decide which side gets each bot
For a human team against the computer, put friends together and assign the bots to the other team. For a mixed match, place bots where they complete the teams you intended to play. Check both the player type and team for each slot. An AI entry on the wrong side changes the matchup even if the total player count looks right.
Check the setup before launch
The official Ashes II FAQ confirms all-human games and games that mix humans with AI. It also announced an eight-player maximum at release. Look at the available slots in your chosen match setup and check your roster rather than assuming every map or team configuration will use all eight places.
FAQ
Can I mix bots and humans in one online match?
Yes. The developer explicitly confirms mixed human and AI multiplayer.
Can I start a multiplayer game without bots?
Yes. All-human online games are also supported.
Can all human players team up against bots?
Yes. The FAQ confirms co-op with friends against AI. Put the humans together and check that the bots are on the opposing side.