How many bottles of wine for a wedding of 120?

You want, at least, two glasses per person = 240 glasses. 1 case = 12 bottles = 60 glasses. So you want 4 cases of wine, at least.

12 bottles is about 60 glasses, so I'd go 30 to be on the safe side. That's about 2 and a half glasses per person.

Usually four 750 ml bottles

It depends on how you much you want and how big the glass is but for normal glasses, its about five.

Two 750ml bottles of wine equal one magnum of wine. One magnum of wine is 1.5 liters, or 1500ml.