When I masturbate several times in a row there is no semen. How come?

If you masturbate a few times in a row the fluid supply from the prostate and seminal vesicles becomes depleted. You can have the great feeling of orgasm, but without ejaculating. It’s called a ‘dry orgasm’. When you masturbate again the next day there is a new supply, making you ejaculate as usual. So don’t worry. You never run out of semen: there’s a fresh supply every day until you die.