It depends on the type of earplugs you have; if you’re unsure, read the instructions provided to see whether the manufacturer advises reuse.
Typically, most foam ear plugs aren’t made for reuse, whereas moulded earplugs can be used again, but you should clean them each time to prevent bacteria building up.
When you’re reusing ear plugs, make sure that they return to their original shape after you pinch them - if they don’t fill your ear properly, they won’t protect you against loud noise.