You mean foam earplugs? You absolutely can reuse them.

You can't reuse them if they get wet (they don't stay compressed when you roll them up - a necessary step before inserting them). And I throw them out if they get too dirty.

But I generally get 7-15 uses out of a set of foam plugs before I chuck them. It depends on how much I sweat when I'm using them, and how dirty my ears are.