Can a petri net fire only one transition at a single moment?
Both.

Only one transition being able to fire at a time is known as interleaving semantics. Multiple transitions firing simultaneously is known as step semantics. Some people doing concurrency theory only call the latter real concurrency, and they care deeply about the difference; I'm sure the difference has consequences, but I've worked with Petri nets and I've never run into a situation where it mattered.

In a Petri net editor I used to work on, durations could be assigned to transitions - so these "transitions" represent tasks or processes that take a certain time. Once that is possible, clearly it makes a great difference whether two of them can be firing at the same time or not. But I would say such "timed transitions" really aren't transitions; rather, they are shorthands for a transition (start), a place (executing), and another transition (termination).