Deciding who you would like to invite to your party involves much more art than science. There are a variety of factors you should consider when you sit down to think about the guest list. Take the following into consideration when deciding on the number of guests.
The first thing you need to decide is where you'll be holding your party. Will it be a house party or out at another location? The most important thing about this decision is that it will tell you whether your guest list needs to be limited by available space. Obviously, you want your guests to have fun, and part of that is considering their comfort, seating and room to roam.
Before you go any further, it's important to set a budget for your party, and then decide whether you want to spend that budget on inexpensive refreshments and food for a lot of people, or more luxe goodies for a select group of friends.
Once those decisions have been made, you should consider, based on the type of party you've planned, how much you can manage on your own. If it's a dinner party, are you comfortable cooking and serving a gourmet meal for 15, or do 8 guests seem more manageable? Or, can you afford to hire assistance?
Finally, and many people will say this is the most important factor, you need to consider the personalities of your guests and who will make good conversation together, or be willing to dance, or participate in whatever activity you've planned. Your guests don't need to know one another, but they should be able to socialize in a mixed group if they don't. And, of course, don't invite friends who get along poorly with one another to the same party–that is unless you intend to make their hostility be the main event.