I've never heard this before or thought about it this way. If you need to restructure your code because of your team size, you have a bigger problem with your code than whether it's separated into microservices or not.
"Let's do everything as microservices" seems like a trend from a few years ago that has now somewhat died down. My suspicion is that it was PaaS companies like Heroku pushing people to do this because it made more sense for their products, but I don't have any evidence for that.
Since then, it seems that people have remembered not to solve a problem you don't have. If your code works and it's easy enough to work on it, you don't need to radically change its structure.
Keep in mind that several companies have tens of thousands of employees working on a mono-repo.
> I'm curious to see if there's a clear cut off which dictates that...
There's no clear cut off for anything in managing a software team. It's not a science, and individual situations vary too much to create hard and fast rules.