I don't know if its the same for a shower, but for a wedding, they tell you to expect about 60% (that is the average)...
but like i said, thats for a wedding...
I am hosting my friends shower, and her guest list is about 60 people, but she knows who will not be making it, but they still want an invite (grandparents in different states etc). She also knows a good amount of the people that wouldn't miss it for the world... so we have a pretty good count so far...
just remember, expect the unexpected! Don't be surprised if people show up who either didn't rsvp, or even those who show up who said they wouldn't!