For a long time, my personal record was six, and that was with my ex, on our first night together (it was the day we met, actually). We were quite young at the time, of course (I was 23, she was 29). The penultimate happened, again with the ex, several years later. We had rented a condo on a nearby lake for a long weekend, and for whatever reason, she and I just went ape with the sex. I did not keep an exact tally, but on average we fucked about eight times per day, over the two and a half days we were there.
Didn't matter where we were, either. Bed, couch, middle of the living room floor, bath, kitchen, the swimming pool outside, didn't matter. And except for the time in the pool, I ate her sweet pretty pussy every time. Oh, it was glorious.
When the weekend finally came to an end, she did mention that her cunny was a bit sore, but it was for her a "oh, it hurts so good" kind of sore. What I didn't tell her was that the underside of my tongue was killing me. This had happened to me a few other times after performing a lot of cunnilingus, but this time was a record, and it took a couple of days before my tongue recovered. But oh, we had such fun that weekend.
And to this day, that remains my records for the most sex in a day (even during my swinger days, I never came close to what happened that weekend with the ex).