Order your invitations 3 to 6 months before your wedding date. You need to give yourself plenty of time to assemble, address and mail your invites. You'll need about 2 to 4 weeks to do this without stressing out!
The quantity of invitations you'll need is not the same as the number of guests you are inviting to the wedding. You'll need one invitation for every:
Order matching reply/RSVP cards and be sure to include a self-addressed, postage-paid envelope for your guests to send it back in. You can save a few bucks here by using postcard replies instead of the traditional reply card with envelope.
Seriously consider including a matching wedding map and guest information card that will help your guests with directions, lodging, and parking information (they will definitely thank you later!).
Send your invitations 6 to 8 weeks before your wedding. Think about sending them earlier or later depending on how many out of town guests you have, your ideal RSVP date, and the location of your wedding (i.e. local or destination).