Last season I think it was 2 tickets for the group A matches - or whatever they call them - Man Utd, Liverpool etc. For lesser opposition it's normally 4-6. During the last European adventure the allowance was quite generous because they just wanted to fill the place as quickly as they could - I got 4 for the home leg against Hamburg. I don't think I've seen a game where you could only buy one ticket - that doesn't mean it'll never happen but you'll normally be good for at least 2. There's also a 'loyalty points' scheme now. I've collected some - if anyone knows that they're for I'd love to know!
Hope that helps.