Do I need a sequence diagram for every use case?

Not necessarily. A use case describes a goal or interaction, while a sequence diagram shows messages exchanged over time. There isn't an automatic one-to-one mapping between them.

You might make several sequence diagrams for a complicated use case, and none for a straightforward one. If this is an assignment, check whether it specifies a required number.

If some use cases share most of their steps, you could show them in one sequence diagram and use an alt fragment for the different branches. Think of it like an if/else.

Don't force everything into one picture, though. Once the branches make it hard to follow, separate diagrams may be clearer.

One high-level use case can involve several subsystems. If showing the whole interaction gets crowded, separate sequence diagrams for the subsystem flows can make the design easier to understand.

Use sequence diagrams where the order of interactions actually needs explaining. A diagram for an obvious flow can add clutter rather than help. They can also stand on their own or accompany user stories, not just use cases.