Do you need one adult for every 3v3 pitch?

For the current England Football 3v3 setup, the guidance is one pitch facilitator per pitch. I would stick to that. It does not mean you need four UEFA-qualified coaches. The facilitator’s job is mainly to keep the pitch safe, support the children with the 3v3 rules, give positive encouragement and make sure the next game is ready to start. If you have one experienced coach and three sensible volunteers, you can brief all four together before the first round. The experienced person can also act as the matchday organiser across the area. Trying to watch two pitches sounds manageable until both need help at the same moment. I’d recruit and brief enough adults for one per pitch, then keep their job deliberately simple.