Based on the sort of crowd you are welcoming, a certain number of guests will not bother to reply even if given a deadline. It is very proper to call all of them for an answer. If you are unpleasant with that, but you need a particular count, a gentle approach would be to ask your potential visitors if they received your request. Most people will apologize profusely as well as quickly offer up the information you will need. How To Write Formal Invitation You then save face to them and you by explaining that you will be only calling because all those pesky caterers are annoying you.