Honestly the rule I put on it nowadays is, if it doesn't have a logical boundary within the organization then it doesn't need to be a separate service.

If you have 6 microservices and they're all being maintained by the same team of developers, you don't need 6 microservices. At most, I would grant maybe 2 - and that's simply because usually once something like payments and credit card handling get involved, for compliance reasons you want to really not touch it unless you have to - but that's still an organization boundary, just a self-imposed/PCI compliance based one.