There seems to be a lot of studies showing that the number of sexual partners is important in determining success of marriage

I don't have the links to any but I have seen a lot of it on TRP - these do seem to be legit studies

I'm not married or looking to get married (probably ever) so don't particularly care about the marriage aspect.

But I do have a GF and so I'm just wondering whether "n-count" has any affect on how likely a long term relationship is to succeed.

Are these statistics bogus or do they hold some value?

Im not even sure this is entirely a RedPill concept, I've actually seen this idea on very "bluepilled" sites:

http://www.dailymail.co.uk/femail/article-4085758/Experts-reveal-sexual-partners-ve-determine-likely-DIVORCE.html