Bumped out of curiosity. I'm new to siloing local sites also.

I would never base a url on any keyword count guideline. I would make a url for what the page is.

That said, if you are talking about a site that is sort of a directory of sorts, like your example,
it is redundant to write plumbers.com/plumbers-in-atlanta. A visitor would know
plumbers.com/atlanta was most assuredly talking about plumbers in Atlanta. Not only
does the visitor know, but it makes silo-ing easier to keep track of.

If the domain did not have the keyword in it, like craigslist ot something, then I would
go for

pipefixit.com/plumbers/georgia/atlanta

Does it matter for SEO? It could in the long run, as your huge urls would be muddled,
buried, and hard to keep track of.

Short site, different type of website, probably not.

Paul

I would probably just replace "plumber" with "plumbing" just to minimize repeating the keyword, but at the same time, covering both variants.

I'm not sure if "residential" is necessary. Depends how much volume there is.

Keep the location name and remove "in".

If you are in confusion then make your URL more clean by just placing categories like this :
1. If its in Alabama - Charliesplumber.com/albama

Not like this :
1. If its in Alabama - Charliesplumber.com/residential-plumber-in-alabama

People will know from the domain name itself about the services you will offer.

But, if you are serving your business both for Residential and commercial purposes according to states, then its better to have a URL as

1. Charliesplumber.com/residential/alabama
2. Charliesplumber.com/commerciall/alabama

URL's being SEO friendly or you can say easy to understand/read URL's usually depends on the categories you have and then in a particular category what product or service you are providing.

Syntax :
Domain name/Category/Product or service name.

Correct me If I am wrong somewhere.

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I would recommend to replace plumbing with plumber. Or you could simply remove that word.