Can't remember if you asked this on the other thread but I'll answer here in case you've not.
If they're geographically close (metro area as an example). It's not too bad but national sites can require some more work. With a metro site all your pages help build local topical relevance whereas you don't really get that with a national site.
On one hand one site means less money on domains and possibly links etc. But single city sites might have a little more location authority since the domain is likely PMD/EMD and all your content is about that location.
I don't use expired domains, it's hard to find non branded ones and the generic ones tend to be spammed from marketers (likely older projects). Just stick with a generic natural sounding fresh domain.
I actually think metro sites are the way to go nowadays. You get to cover a large population, you can rank in easier places before your main location page and you get to build up some location authority.
https://en.wikipedia.org/wiki/Metropolitan_statistical_area
Where you pick really depends on your budget and experience. You need enough population but you also don't want super hard main keywords (unless you're prepared for it). In the harder areas it's likely competitors will be building links and content to support other location pages but in many metro areas you might find they either don't target those surrounding areas or they're pages are fairly weak (content or links).
10 random US cities will likely be harder to rank then 10 places within the same metropolitan area since internal linking makes sense for metro sites and all the locations are fairly well related entities so help with authority.
