Ditto PP's...though your mom should be prepared to make phone calls for an accurate headcount, so be sure to give her a list of phone numbers if she doesn't have them.
For some reason, most of the events I have had not only did most not RSVP... the ones that did would RSVP to someone else! (aka- MIL or my sister) I guess they were fine with showing up to the event to get free food, etc... but couldn't talk to me on the phone for two seconds. Rude!