How many people can you expect at your event? This depends on a number of variables.
1) The host's history of hosting and attending parties. If you try to throw the same party every month, you can expect a low Rate of Return because there's little anticipation. Likewise, if you're a bad guest (constantly flaking out on others, not RSVPing but then showing up, etc.), expect your event to be low on the priority list of others.
2) Guests' other options. If it's New Year's Eve, you'd better come up with a hell of a party if you expect the turnout to be significant.
3) Amount of notice. This is a given. People are busy, so the earlier you can get on people's calendars the better. That's why save-the-dates for weddings have become so popular in recent years.
4) The event itself. Is it a fantastic bash on the beach or a simple Gray's Anatomy viewing? What do your guests like to do? On a related note, does your invitation make people excited to come? Thinking about recent events I've had at home, our annual rooftop party had an amazing RoR. We sent the Evite out over a month in advance because I know what summers in the city are like - everyone is booked weeks in advance. Also, this is a party we only do once a year. In fact, I do no more than two huge parties a year. Any more than that and they become old hat. On the other hand, our little New Year's Eve potluck dinner had a purposely low turnout. We sent the invite out just two weeks in advance and didn't hype it too much beforehand. It's an exhausting time of year for an event planner and fewer guests meant an easier party!