In a study of ACL reconstructed patients, those that could do at least 22 single leg rises had better knee-related quality of life at 1 and 3 years post-op than those that couldn't do 22 reps (Culvenor et al, 2016).
Although this information is directed at ACL patients, the ability to perform single leg squats can improve physical performance in all aspects of life for EVERYONE!
So share this around and test yourself and your friends. Set the bar high, and try to be within 2 reps of your non-dominant leg. Like always, watch the QUALITY!! Don't "flop" onto the bench/chair, and don't let your leg collapse inwards.
I can only only do 10 reps on my R) side and 7 reps on my L) side, I have a lot of work to do!!