You can create a crew of 6 consisting of 1 trainer and a maximum of 5 Chibi Frens. While you can run missions with a smaller crew (at least 1 trainer + 1 pet) you have a higher chance of reaching a better result with a bigger crew. Not only the crew size but also how you've formed your crew will influence the mission outcome (hint: homogenous vs. heterogeneous teams). During the beta, you will be able to create a maximum of one crew. However, you can always readjust your crew after a mission.