My opinion on this has evolved considerably over time, toward the lower end of the scale. Somewhere in Navis Primer it mentions that the largest privately-owned fleet belongs to a Navigator House and numbers in the 40's.

I've enumerated the number of IN ships in my campaign, and BFK has 14 capital ships (half CLs) and 28 escorts. This will vary in the course of the campaign. I don't consider this normal or average. I go by the 50-75 number, of which 1/3 are capital ships. In most sectors that means about 25 capital ships, of which 5 are BB's, 5 are CL's, and 15 are CA's, plus 50 escorts. I have more CL's in BFK given its primary mission.

There are approx. 5000 sectors in the Imperium, if averages hold true (and what else do we have to go on?), that gives the Imperium something over 300k warships of which 100k are capital ships. Seems about right to me, though I think the ratio of capital ships is high. To me that just means there are lots of system ships out there, especially in populated sectors. I can live with this figure.

Yeah, I'm sure Winterscale has a lot of ships, but I expect that a number of them, even a decently high number, compared to the total, belong to other people, and they pay him a tithe to "fly under his colors", if you will. He owns the people, to some degree, and gets a share of their various endeavors, while telling them what to do, from time to time, when he actually cares. That all said, he doesn't likely "own" that many, nor pay the upkeep on them, but helps those who serve under him maintain their ships, in his name. I presume most other RT's do something similar, at least paying other captains to perform services, that their immediate resources can be directed elsewhere. This all might help explain better how people in the Expanse, who otherwise seem so much more disposed to just killing the competition, and taking their stuff, actually work together, "get along", and prosper. I do stuff for Winterscale, his infinite money makes my cruiser's upkeep easier , and we both make a pretty profit (factor). I can do individual tasks for him, possibly without attracting attention he's involved (this is why George Lucas lied to the studio when they were making RotJ, so they wouldn't nickel and dime him death, for his obviously gonna be big movie), while his contacts and informants can give him, and thus me, ideas on lucrative places to go. Win win.

A few factors that could raise or lower the size of BFC:

The Calixis Sector is repeatedly pointed out as being far from the massive armies and navies of worlds closer to Terra. It is quite isolated by Imperial standards, being on the wrong side of the Eye of Terror.

Being at the fringe, it is less important politically so has fewer advocates for massive armies.

As a waypoint for the Achilus Crusade, it gains importance if only to secure the supply lines.

Nova cannons are rare in the sector, since most vessels with them have been reassigned to more active warzones. Other warships have been noted as been reassigned after the Angevin Crusade.

Only two battleships has been named in the sector, and those might be two names accidentally given to the same vessel (both "flagships").

The battlefleet command are worried about their capital ships being shipped off to Jericho, to the point they've reassigned at least one battlecruiser to BFK they wouldn't have otherwise.

Personally, I see Calixis as having fewer than average big capital ships. There simply isn't the justification to have a massive supply of them in a non-warzone, especially with the ongoing drain of ships into the Jericho Reach.

And if the fit really did hit the shan, there are enough full-on Inquisitors there that, should they deem it necessary, they could get several big ships to bail them out. Don't leave them lingering, just because you MIGHT need them; 40K isn't, so much, the world of deterrent through threat of destruction, when you can just destroy much of what you want, with no need to justify it.