While I admit that i'd prefer a girl to have a lower sex count than not, as long as she is sexually satisfied with me as a partner and devoted to the relationship it's not really a concern until STDs come into the pile. I suppose i'd wonder if a girl has any sort of standard if she has managed to fuck 50 guys in her life, which might not say a lot for me as her 51st.
It's really just a matter of feeling like you were more the one he/she was waiting for, but the concept of men sleeping with 30 girls only to shame the girls they then fucked is actually really funny. Personally for me, people who like to reserve their sex drives for people they deem special are charming as long as it's not for religious purposes, but that's pretty rare these days.
That said trying to put a strict number on it is foolish, looking at the core of why it bothers you at all is the important part.
The world would be so much better if open relationships were more common, if attitudes toward sex for the purpose of pleasure were more relaxed. Greed and insecurity and pride will prevent it from ever becoming more than a niche lifestyle, however. It's a shame.
If you're wanting a monogamous relationship, it doesn't matter how much sex anyone has had before agreeing to monogamy, only who they have it with after that. If it's a concern,it's a concern because of your own faults, not the other person's, and I would think they'd be better off without you anyway.
This is also kind of too broad a statement to make in my mind. The core reason people want exclusive rights to their husband/wife to be is not just the sex but the rest of the things they can contribute to improving your daily life. Money, kids, cooking, humor, whatever it is couples aspire to share, most would agree there is just not enough time in the world to do all we'd like to do in one day, especially if you have a family. The idea of jeopardizing the family unit so you can get your nut off in a different hole is hardly something i'd consider 'better for everyone', unfortunately I do understand the instinct of being attracted to many people as well.
If you can say you don't ever want to be married, have kids, and have thought through your life from now until the day you (hopefully) die of old age and still think yes, open relationships are a perfect solution, that's pretty nifty, but I can't imagine many people can balance a healthy family life, raise kids and balance all life has to offer while deciding to fuck around with many people. For people who desire the 'American dream' I consider open relationships very destructive to that goal, but if you have a different goal in life and can make it work, that's cool too, but most people don't have self control. I would actually be really interested to read what a person who is within open relationships plans for their long term life vs the typical ambitions of most people as I'm sure it would be interesting, possibly enlightening, but I have yet to run into a swinger in their 60s either