I was watching Ireland am and they said according to a survey (I can't find which one) that people over the age of 35 think that once your count of sexual partners/ lovers/ flings what ever you want to call it reaches 20 it becomes a concern.
I don't think that number sounds too high for a single person at 35. I was with my 1st bf for 2 years (16-18) 2nd bf (18 - 22) then met DH at 23 and would have had a few shorter term things so my 'number' is low. I was DH's 1st serious relationship, he probably has a much higher number IDK what it is.
I have a few friends who have been pretty much single their entire adult life. I can see that hitting the 20 mark could happen pretty easily, if you lose your virginity at 18 that's really not many at all. These are 'respectable' professional women (actuary/ solicitor/ accountant) who just happen to not have met the right man and like a bit of sex!
YABU - More than 20 is cause for concern
YANBU - 20 really isn't that high