Your caterer will need both. The estimate is for your quoted/estimated price for the caterer's services. You will need the final head count before the wedding to give to the caterer so they know how much food to make and what your final bill will be. My husband and I had to give our final headcount about two weeks before our wedding and we paid the balance the day our caterer sent us our bill, so at the wedding we didn't have to worry about paying anyone anything - we paid everyone within the two weeks before our wedding so we only had to tip our vendors the day of.
It sounds like you are asking about when hiring a caterer. No, when you are looking for a caterer you just need a decent estimate that you expect to have at your wedding. They will quote you based on your estimate (usually a per person quote). You will contract with the caterer and decide on all details and will then finalize your count with the caterer you’ve hired about a month prior to the wedding.