The minimum number of satellites to lock is 6, but it also uses a number of other quality metrics, including GPS velocity accuracy, GPS horizontal accuracy etc.
It will get GPS lock much faster if you connect with the app first (or the status interface of the web interface), as the app will download both the time and a block of GPS assistance data to the drone which reduces the lock time a great deal.
You can fly without GPS lock (in indoor mode), but if you want to fly with GPS lock then it does need a high quality GPS lock otherwise it would fly very badly.
Cheers, Tridge