How much alcohol do we need for 120 guest?

There's an app for that! It's on Pinterest a lot. Look for liquor or wine calculator. The amount also varies with the time of day.

Like @Angel said, there are a ton of alcohol calculators online. Try a few out and see what you get! To be on the safe side I'd budget 2 drinks per person per hour

BevMo has a good calculator

http://www.bevmo.com/drink-calculator/

Ditto PPs. But really I came to say *guests.

There are several guides you could go by. Just find one that is easy to understand. Most recommend 1 drink per person per hour factoring in the non drinkers. So a typical bottle of wine is 5 drinks, and each beer is one. So multiple 120 times the number of hours in your reception.That tells you how many drinks as a minimum you will need. So if it's 4 hours you need 480 drinks. Depending on if you have more wine or beer drinkers will depend on how much of each you buy. So if you split it in half you need 240 beers and 48 bottles of wine.

Buy a bit more than you think you'll need! Try to buy from a business that will let you return extra bottles.