These two questions are the same question: the number of pitches determines how many matches can run at once, and therefore how long your tournament takes.
duration ≈ (number of matches ÷ number of pitches) × (match length + change-over)
The change-over is the margin between two consecutive matches on the same pitch — for warming up, switching and entering the result. Count on 3 to 5 minutes; forget it and your tournament will run structurally late.
12 teams in 3 groups of 4 gives 18 group matches. With 12 minutes of play and 3 minutes change-over:
Add a final stage of 4 matches and count on another 20 to 30 minutes — those matches parallelise less well because they wait on each other.
A team can only be on one pitch at a time. With 3 groups and 4 pitches, the fourth regularly sits empty because there is no match that can be scheduled without a conflict. Rule of thumb: roughly one pitch per group runs smoothly.
Think about rest too: do not let a team play two matches back to back. In UwTornooi you set a minimum rest for that; matches where a team gets too little recovery turn orange in the schedule.
Three is the comfortable choice with 3 groups of 4: each group gets its own pitch and the 18 group matches are played in about an hour and a half. With two pitches the same tournament takes well over two hours.
With 2 groups of 4 that is 12 group matches. On 2 pitches, 12 minutes of play and 3 minutes change-over: (12 ÷ 2) × 15 = 90 minutes, plus a final stage of 3 or 4 matches.
Put a break window on that one pitch instead of on the whole tournament. The rest play on and the generator schedules nothing there.