Tarantulas don't actually become pregnant. They can lay eggs whether they have mated or not - but if they have not mated, the eggs will be infertile and will not hatch. During mating the male deposits his sperm in the female's spermathecae (basically sperm storage receptacles) where she retains them until she is ready to lay eggs. She can use those sperm to fertilize her eggs at any time until her next molt. (During molting, she sheds the lining of her spermathecae - along with any remaining sperm.) Some tarantulas can even lay multiple egg sacs from a single mating. A single mating is all that is required for the female to lay fertile eggs, but to increase their chances of a successful mating, some people will try for multiple pairings - particularly if they don't actually witness insertion. Just because the male and female have shared an enclosure doesn't guarantee that they will mate.
If the male is not killed/eaten by the female, he can be mated multiple times. There isn't really a set number of times that he can mate, but he is not fertile for the remainder of his lifetime. Peak fertility may only last a few months after his ultimate molt.
A mature male can live for up to a year or two after his final molt - but not all males live that long. Some will die within a few months of their final molt, regardless of whether they have mated or not.