The limit on a futsal tournament is almost never how many teams sign up — it is how many pitch hours you have booked. Work backwards from the booking. A futsal slot with two ten-minute halves, a break and changeover runs about 25 minutes end to end, so two pitches give you roughly five matches an hour, and a six-hour Sunday gives you about thirty.
That number kills the format most organizers reach for first. Twelve teams playing a full round robin is 66 matches — nearly fourteen hours on two pitches, or three separate Sundays. Splitting the same twelve into four groups of three and running quarter-finals onward brings it to 19 matches and finishes inside four hours, with every team still guaranteed two group games before anything is at stake.
If you want more guaranteed football, three groups of four costs 25 matches and about five hours — still one day, and every team plays three. That is usually the right trade for a one-day cup: enough games that nobody drives home after twenty minutes, few enough that you finish before the lights go off.