Yes, callbacks are generally just for leads. However, I once had one just for seeing who could dance because they didn't have us dance at the first audition.
Usually if the director/s know that they want you at least in the ensemble, but you don't fit a lead, they will not call you in again. If they don't call you and aren't posting a cast list you could always call to double check whether or not you made ensemble.
I say that because my first time out I had a callback, but I didn't receive a call for ensemble because they left a message on someone else's answering machine by mistake! Thankfully at the callbacks the director told us that we all would at least be in the chorus, or I would have missed out on being part of a great production.
Currently~ Miss Myrna Thorn in Ruthless! the Musical
Recently~ Emma Carew (Jekyll & Hyde), Sister Mary Leo (Nunsense), Demeter (Cats), Tzeitel (Fiddler on the Roof)