I agree to Ganesh Sanap, it would be way lot more easy if users you meant to send an email is inside an O365 group, you will just have to put 1 single email (groups email) then it will send an email to every users that is a member to that group.
Anyways, If the process or your case of using an SP Group is inevitable, it would be better if you send more information or screenshot of the workflow action on how you are doing the setup so we can see whats happening in between. Base on the case, I GUESS, its an issue of a query on the SP Group but only retrieving 60 persons, somewhere in the request we could increase the no of results if you have included a $top parameters in the rest URL or we could have a recursive wf that will loop until users in the group will be retrieved.
Hope the concept helps! Happy Learning!