There are very clear cut definitions provided in the code and regulation on each of these, but the simple examples above are a good way to think of it.

But if that just is not in the cards, then you need to figure out first what (if/any) STR regulations come into play as you go to that size, as some areas would end up classifying that as commercial in nature and you might have other things to manage with respect to that. 

Did regulations change?

La Quinta just put a 90 day moratorium on new STR permits, so expect some tightening of regulations.

Any restrictions regulations?

They regularly increase regulations, making it harder and harder to be a Landlord.

@Richard Jeffries The only way to venture into the Nashville STR market with the amount of regulation in place right now is to buy a property in commercial zoning.

It's a bit more regulated here in Canada, so not so much experience yet.

Different cities have different rental regulations (rental license, inspections, etc), and also if there's an HOA, they have their own restrictions but nothing currently statewide.  

Take a look at city and county regulations sometimes they require a property manager if you're more than x number of miles away.