Choosing how much alcohol to serve at a backyard wedding is often one of the hardest decisions. Too little, guests won’t have a good time. Too much, guest go c-razy and you get a lot of nipple shots at the Smile Booth (not always a bad thing).
Party Central has a great guide to serving alcohol at parties. Keep in mind that this is for a full bar, not a limited specialty bar. I’m also assuming that this is geared towards an evening event. If you do end up buying more than you need, check with the supplier ahead of time. They may let you return unopened bottles of wine and alcohol.
Other Resources: Liene at Blue Orchid has posted a great guide to alcohol (part of her 30 Days to Planning a Wedding feature). I also found a drink calculator at That’s a Wrapper, including this break down of drinks per bottle.
The legal stuff: Keep in mind that with a DIY bar, you take on a lot of liability. Make sure your (or your parents) home owner’s insurance covers a party, and you have en experienced bartender who can set limits on guests.