You will need to know who to expect at the Bar or Bat Mitzvah about two weeks before the event. The caterer will need the numbers and you'll want to finalize the seating arrangements, party favors and last minute details. The thing is that many people will miss the deadline you give them, just a fact of life! So I suggest you ask for responses three weeks in advance of the big day. This way you have an extra week to collect any stragglers or make some follow up phone calls if needed.
Usually you will include a separate reply card along with the invitation. This will have a place for your guests to fill in their name and check off whether they will be able to attend or not. Lately some people are choosing to not include a separate reply card and simply have guests email or phone in replies. This is a great option if you would like to keep within a smaller budget, your event will be a little bit more casual, or if you prefer a super green alternative. If you choose to go this route, be sure to keep track of all responses, on a handwritten list or an excel spreadsheet, whichever is easiest for you.
I do so still love a separate reply card though for a few reasons. This is after all a very important event in your family's life as well as an occasion with lots of tradition attached to it. So a reply card that is filled out by hand and mailed is in keeping with this. You may also be surprised to find some sweet messages handwritten on some of the reply cards and these may become lovely keepsakes.
Whichever way you choose to collect the responses, don't worry, you really can't go wrong. And once the replies start coming in the excitement really begins. Enjoy this special time! This is the good stuff.