Is it a significant occasion? If so, I'd be buying something bubbly (exact kind of bubbles will obviously depend on your budget!), as well as white and red wine. Probably half a case.
General rule of thumb is that a bottle of wine = 6 glasses; however, if you have large glasses (like the ones they pour in a pub if you ask for a large glass of wine) this will only yield 3 glasses.
Do you have a Majestic Wine near you? They do 'sale or return', so it means you can buy more than you think you might need, knowing that any that's unopened can be returned.
I'd go for a case of white wine and a case of red. If you're buying bubbly as well, I'd go for a case of white, half a case of bubbly and half a case of red. Generally most people seem to drink white - but if you know you have friends who are mainly red drinkers, you'll obviously need to change this!
Don't bother buying spirits - too expensive, and you'll also have to provide mixers.
I'd also be buying some beer/lager - again, depends if you know your friends are likely to drink those. You can generally buy boxes of 24 bottles; I'd probably buy 2 - one of lager and one of beer/bitter/IPA, etc.
Then soft drinks - again, this depends on knowing who your guests are and what they like to drink. But I'd usually have some fruit juice, a large bottle of lemonade and large bottle of Diet Coke (or similar).
Remember that your guests are also likely to bring drinks with them - so you may wish to cut down on the quantities I've suggested! But I'd always far rather have too much than worry about running out. And if you can get the 'sale or return' deal, then you will get your money back afterwards if you bought too much.