If you have a GFS near you, they have these really really large bags for like 2.99. We got 4 of them for my DSs open house. We had 300 people, but we also had lots and lots of other foods. If you are only have sandwiches, chips and desserts, I would think you will need 10 bags of the GFS ones and maybe 25 of the normal size ones.
The problem with the normal sized ones is they keep lowering the amount of chips you get in a bag. 12 years ago I worked for a snack company and our standard size bag was 16ozs, now I think they are around 10.