If you click on the name of a recipient in your list, the edit popup will open.
Here you can set a global number of guests for the single, e.g. 1 and couple/family, e.g. 4 setting.
If you choose the single setting, this recipient will display the guest number 1. For couple/family it would be 4.
If the number is 2 or larger, then the recipient will have the option to edit this number on the reply page.
If you e.g. invite a family of 4, but only the mother can make it, then she can accept the invitation and change the number of guests to 1.