Headcount refers to the number of adults responsible for paying the rent. The one week non-refundable Reposit fee will then be split between them.
E.g A family of 4 (2 adults and 2 children) rent a property with a £1050 rent per month. As there are 2 adults responsible, the headcount will be 2. The Reposit fee will be split in 2 meaning that each person will pay £121
There is a handy calculator on the website that does all the work for you: https://getreposit.uk/tenants/