You can run out of semen, let me put it that way. Sperm only makes up a small fraction of semen, and after a few ejaculations in a short period of time, there will be no more fluids left to make semen. Some men seem to be able to replenish more frequently than others. Most younger men can easily ejacuate every day and many can sustain twice a day; some can ejaculate 6 times in the span of a few hours, though it is rare they can sustain that day after day. Some will still have sufficient ejaculatory fluids but will be either too tired or will fail to be sufficiently excited to ejaculate again in a short time period.
This post reminded me of an article i read. Some tertairy student in German broke the world record for masterbating the most times in 24 hours. It was 36 times. So some people must have really quick sperm regeneration.