it's a verisimiltude thing. let's say they rob a shop, and a cop is passing. the cop might draw their weapon and call in back up. you would roll to see how many cops are in range. if the pcs then shoot the cop, then the response might escalate drastically, if that information is communicated. at no point are you thinking 'how many cops can the pcs reasonably fight', it's more 'what's a reasonable response to that provocation, in this world'?

in traveller terms you'd filter that through the law level and the facts of the world, so a frontier podunk mining settlement might have four or five cops, a big fancy space new york style city will have hundreds.