Yeah, I actually agree with you (more or less) in terms of gross availability of hulls and crews (although I suspect there are far more hulls than there are officers and specialists to crew them, given the existence of the Reserve fleets). I was more quibbling over terminology. I guess it's like the arbites being used as a generic term for law enforcement, despite the fact that most of the organisations and persons so described are not members of the Adeptus Arbites. With that in mind (and while there is some extrapolation from canon - Passage Watch 27-Est for example - I must admit this is guesswork), a Battlefleet is not technically a Battlefleet unless set up to combat a specific threat, but any formation of warships may be colloquially referred to as a battlefleet, even if they technically have their own permanent designation.
I forgot about the reserve fleets! My thinking on reserve fleets is that they don't so much consist of ships that the Navy doesn't have crew for, (but that's a possibility) but to a greater degree consist of ships that are not economic or safe to operate except in cases of dire emergency. A lot of the Mid M35 "chaos cruiser" and grand cruiser patterns are heavily implied in a number of sources to be inherently flawed designs that tend to warp and corrupt their crews, and as such are not well regarded. Grand cruisers are only slightly more powerful than battlecruisers, but require disproportionately large crews to operate. Plus, reserve fleets in space don't operate like reserve fleets in the modern, planet bound world: spaceraft don't decay in a vacuum. They don't rust or fall apart. Kept in deep space reserve yards, they are effectively immortal. So it's probably more trouble to attempt to dismantle a ship than it is to just leave it sealed up in a naval yard for centuries.
er...so where was I going with this again? Sorry, got carried away... Ah yes. Ship numbers. I'd completely forgotten about reserve fleets. What do you think represents a reasonable size for a Sector reserve fleet? 10% of the size of the rest of the "active" fleet? 20%? Somewhere between those two sounds about right... so in the case of a 200 strong Sector Fleet, this would mean 20-40 mothballed ships of various classes, probably with a higher percentage of capital ships than the fleet proper... Any thoughts?
There will be escort squadrons too; albeit ecclectic ones. This will be the source of a lot of the frigates and destroyers operated by rogue traders - "c'mon, you're not using it, are you?" - and will get topped up in that any ship the Navy buys in as a prize from its frigate captains (which would mostly be raiders operated previously by pirates), that can be mobilized as an expendable reserve in time of war.
But yes, about 1/4 again as a stood-down reserve sounds fine.
Equally, of the 'active fleet', you can probably work on 1/4 of the fleet stood down for resupply, refit and maintenance at any one time