I personally think three flags is best, but it depends on a lot of factors, such as the size of your community. If you have a relatively small community, then two flags may be best. But if it is larger, then maybe more than three.
It also depends on how many moderators you have too. If you only have a few, then maybe lower that benchmark. If you have a lot, maybe raise it.
All in all however, every flag ends up in the review queue for staff to look at. You can have one flag on a post, and then a moderator will hide it without reaching the three flag benchmark 