NBR: How much cake should I buy?

MrsAJL

This weekend my organization is providing a luncheon to a boy who will become a bar mitzvah on Saturday.  I'm donating the cake.

The boy and his parents said they just wanted a sheet cake from C o s t c o. Can do.  But here's my dilemma:

1) The sheet cake serves 48.

2) The bar mitzvah family has invited 60 people to the event.

3) There will be at least 20-30 more people attending because the bar mitzvah happens as part of Saturday morning services and members of the congregation are invited to the kiddush luncheon.

The Question:  Do I buy one cake, expecting that not everyone will eat a piece or we can cut the pieces small?  OR Do I buy two cakes with the expectation that everyone will take a piece, but if not we'll have a lot of leftover?

The cost is $17.99+ tax/ cake, but I'm not concerned about that really. Just put it in there in case some one asked.