It all depends. At least 15 but if those opponents are outside their range, it could need more. If a picture has won and lost 10 times against pictures in a 8-9 range and has won 50 times against 4s, those 4s aren't taken into account. They don't mean much since we are already pretty sure they are in the 8-9 range.
Also if a picture has won 100 times against 4s and lost 100 times against 9.5s... That doesn't give us much information either. We need 15 battles within a tight range. Even if we have 100 wins against 7s and 8s and no losses, that doesn't tell us much until they get some wins and loses in the 8-10 range.