The short answer is that it depends on the person. Some people hook up with friends, some people hook up with people they’ve just met. Some people have sex with/make out with the same person on an ongoing basis, for others it’s a one-off thing when they’re drunk. Some people - quite a lot of them, actually - are entirely (or almost entirely) celibate when they’re not in a relationship, others pick up a new person almost every week. There are also a whole lot of people out there who are make-out happy, but who had sex with only a small portion of the people they’ve kissed.
But that’s probably not what you’re after, so I’ll give you a few examples of common patterns amongst people I’ve spoken to:
1) Hooking up often, but with the same person/people. “Hook up culture is 100% true,” one of my recent interviewees told me, regaling me with tales of college parties “where everyone slept around.” I queried her on this, pointing to statistics that suggest that the average college student sleeps with four people over four years of university. Did she think those statistics were wrong, I asked her - or at least not in line with her experiences and observations? No, she said, they were right too. She explained: “You would have sex with the same person consistently for a season, and you would have sex with them casually. … [I]f the next weekend, if you slept with someone else, that would be a big drama. But it was still very casual sex. It wasn’t like you were doing it because you were in love and wanted to get married.”
2) Sporadic, “seasonal” sex. Not as in “I only have sex in the spring”, but lots of the people I’ve interviewed go through phases. Maybe they slept with a bunch of people they met in bars (or on Adult Matchmaker) in their first year at university, but stopped having casual sex soon after. Maybe they’ll have a couple of one nighters in the aftermaths of a long-term relationship. Maybe they’re long term singles who have a two-week fling once or twice a week. I’ve also spoken with quite a few people (particularly on the younger end of my spectrum) whose only experiences with sex have been casual and sporadic – and often alcohol fuelled.
3) Making out with lots of strangers, but not going further than groping on the dance floor.
4) Not hooking up at all. Statistically, they’re a minority, but there are still more 20-something virgins out there than you think.
5) Having sex in monogamous dating relationships, either primarily or only. According to data from the2008 National Longitudinal Study of Adolescent Health (USA), the most common sex and relationships pattern is not actually casual sex, but serial monogamy.