At the party= number of attendees at a party(event) In the party= number of people in a group eg hunting party, school trip etc also it could mean the number of guests for a restaurant reservation.

hello, "at the party" means attending a party. "in the party" means the number of people in a group, as in a restaurant. (Hello, I would like to make reservation." "Yes, sir. How many people are in your party?") Hope that helps, Martha

If by party, you mean a party where friends get together for fun, you would use the word "at". But if you were to use the word party in another context (like search party) you would use "in". Obviously, a search party is a much more rare use of the word though. So the most common answer is "at".