Depending on who you ask the answer is either "It depends completely on the task, and it is in any case much more important that you divide your application in the right places than exactly how many bits you end up with", or "1".
If you ask Amazon then the more the merrier, because the number of microservices is effectively a multiplier on the bill.
I sometimes wonder if there could be an optimal number of microservices. As far as I know no one has connected issue data to the number of microservices before. Maybe there‘s an optimal number like „8“ which leads to lower number of bugs and faster resolution times.