How much beer for my party?

I am hosting a birthday party/bbq at my house this weekend. I will be making a sangria but I also want to have beer available for anyone that wants it. The question is how much beer to buy. I’m expecting 45 adults in their mid-30s to mid-40s. The party starts at 5 pm and dinner and cake will be served. This is a family friendly party and I don’t think anyone is looking to get sloshed. Thanks in advance!