A majority of the fluid comes from the seminal vesicles and prostate (90-95%). I don't think it matters if you run out of sperm, but rather the rest of the fluid, since people with their tubes cut, can ejaculate just fine. I can kind of feel the prostate (or seminal vesicles?) kind of fill up as I masturbate. If I ejaculate a few times in a short period (maybe 15 minutes?), then I might run short of 'fluid', but it just takes maybe ~10 minutes of stimulation to fill up again for me. I do think some guys incorrectly think they're empty because of the tiredness from the refractory period.
I always tell myself "one day I'm going to do this." But after maybe three times in a day, I lose interest in trying. One day I might subject myself to just sitting there with never ending porn and jerking it every time it starts to even feel like it might be getting up again.
I think it was seven....one of those lazy rainy afternoons perfect for not getting out of bed and having sex all day. However, it's pretty much diminishing returns after the first two rounds....the rest was more or less just to do it...by the last one it wasn't really worth all the effort...other than to remember it happened..
Like one other commenter above, I achieved ten exactly once. It was a combination of sex and masturbation, several weeks after my first wife and I started fucking. It was enough time together that we had gotten to know each other in bed quite well. We liked to both fuck and masturbate together, and at the end of one day we had spent almost entirely in bed, she had fone back to her place and we were I talking on the phone and tallied up my orgasms for the day, and it was nine. So with her listening I was determined to cum one more time, and even though it was challenging both because I was spent and my cock was quite sensitive, I managed one more. It was a very small amount of semen, not very thick at all. The closest I ever again came to that mark was six or so on several occasions. Three of four wasn’t unusual until I was in mid 50s. Now at 64 it’s about 3 to 5 times per week.