The guest limit for a party should be made crystal clear. Most of the time you will be inviting a couple to a party so you need to make it clear on your invitation that only two people are invited. Get creative on the invitation and create two checkboxes with 1 - attending or 2 - attending. Do not let your guests control the number of people attending. The party budget and seating limitations will dictate how many guests per invitation will be allowed.
Lack of common sense happens all the time with the people you invite to parties. Avoid misunderstandings by being upfront that there is a guest limit. Remember to indicate that no children are allowed if it's an adult only type of party.