How many characters should be present in URL structure?

I would avoid very long URLs although... I would consider it more important for various reasons to have URL match page title (i.e. URL containing relevant keywords that match content)

Maybe ask in one of the more general forums/sections for more opinions

There's no specific URL structure that need to be followed as a mandatary rule. But you can use Url with no special characters and with contextual keywords in it to help SEO.

You should keep your URL short. 50 characters you can use in making URLs.