Expect to be overwhelmed! The first time we went we only registered for three things! We actually spent most of the time looking at everything and figuring out what we wanted.
Most major stores will give you an idea of what types and quantities of certain things (towels, sheets etc) you should be asking for based on the number of bridal showers and number of attendees and the size and type of the wedding.
Expect to have to go back a few times at least..