15 Script drafts is actually not that unusual for a typical Hollywood movie. Though in Covenant's case it was already at 15 more than a year before filming would start so who knows how many drafts it ended up with.
Sometimes the differences between the various drafts can also be very minor, eg there wasn't a huge difference between the two Prometheus Spaith drafts that were released but there were many differences between Spaith's last draft and Lindelof's shooting script.