Personally, I would look at the area they occupy (in square miles) and then ask how they support themselves (e.g. do they farm? hunter-gatherers?).
If they are hunter-gatherers, maybe see this:
https://persquaremile.com/2011/08/17/hunter-gatherer-populations-show-humans-are-hardwired-for-density/
And increase maybe by 50% because I think hobgoblins are smaller than humans.
If they use agriculture, goblinoids are usually assumed to occupy poor quality lands, so maybe population densities around 10-20 per sq.mile.
Storywise, it has to be a big enough population that Masra can't just wipe them out, but probably you don't want them to outnumber Masra enough to conquer it either (although hobgoblins lack organization, so there's some margin there). So maybe 30,000-300,000. How big a threat do you want them to be? Should it be a disaster if someone comes and unifies the hobgoblins?
Of course the settlement right next to Masra will be smaller than that. I personally imagine hobgoblins having trouble maintaining stable local groups bigger than a couple hundred, unless someone strong is able to hold them together for a while.