Do I need a Collavate Business license to approve/reject files? How many can I approve/reject?

No, you do not need to have a license to accept/reject processes. When you log into Collavate with your Gmail account, you automatically obtain a Team Edition license. With this license, you can approve/reject as many files as you want.  

* NOTE: If you want to submit files for processing or if the approver is the same Google Workspace domain users, you must have a paid license.