BFG manual page 86
Each battlefleet normally consists of between 50 and 75 warships of varying size, although in some sectors this willb e more or less, according to the importance of the sector and the number of enemies it must contend with. As well as these destroyers, frigates, cruisers and battleships, a battlefleet also has access to countles smaller vessels such as transports, shuttles, messenger craft and long range patrol craft. In addition to interstellar vessels, a sector will also be protected by numerous ships incapable of warp travel, such as system patrol ships and defence monitors.
Size of a battlefleet as per BFg. It's more an 'average' than an absolute, since we know both that Battlefleet armageddon and the EoT campaign mentioned larger fleets. Other sources mention considerably greater, but it might be chalked up to 'active' vs 'reserve' vessels (we know the Imperium mothballs its older or harder to maintain ship designs.
This also includes only warp capable, military warships. It doesn't include nonmilitary (or probably quasi military, like armed merchant cruisers or such) and privately owned vessels. Obviously it also does not include sub-stellar vessels of any kind (although since most are generally escort size, this isn't a huge factor either.)
Andy Chambers quote from the net, found here
Q: In the ship section, there is mention of Imperial Oberon-class battlecruisers and Apocalypse-class battleships. Were these cut from the registry to save room? Also, will these be making an appearance in a future issue of WD or possibly a supplement?
A: No we just wanted to give an impression of there being a greater number of ship classes and designs in the galaxy than were depicted in the book, particularly as Gothic sector is but one among thousands. Having said that we might well put some extra ship designs in WD at some point.
Thousands of sectors probably means thousands of battlefleets, at least according to Andy ChamberS (for whatever that is worth) With BFG we might figure on at around 100-150 thousand warp capable starships as a rough figure.
The combined merchant fleets comprise almost 90% of all interstellar spacecraft in the Imperium. Each fleet is based in one of the five Segmentae Majoris. and its administrative staff operate from the Segmcntum Fortress. For example, the Solar fleet is based on Mars, while the fleet of the northern zone - the Segmentum Obscurus - is based on Cypra Mundi. Although these fleet bases are huge ports equipped with docks, shipyards and repair facilities, their main function is to administrate the fleets operating within their area. Only a small proportion of ships ever travel to the Segmentum Fortress where they are theoretically based.
Although the vast majority of interstellar spacecraft are part of the merchant fleets, there are several thousand ships registered to individuals, families or trading cartels. All privately-owned interstellar craft operate along routes licensed to them by the fleet authorities responsible for shipping within that Segementum.
Space Fleet. Older lfuff but may still be valid in some cases (it has been recycled in Rogue Trader's RPG for example.) Space Fleet divided the navy up for each segmentum into three components - military, merchnat and civilian (privately owned) vessels. 90% of the naval ships were merchant vessels, and clearly less than 1% are going to be privately owned. That means around 9-10% are military vessels.
3rd edition rules, page 100
he psychic beacon stretches across 70,000 light years of space, a focus for the millions of commercial craft and warships that navigate through the tumultuous waves of the warp. The Navigators who guide human ships need the Astronomican, and without it the Imperium would fragment as warp travel became utterly hazardous and Imperial authority would be impossible to enforce.
'millions' of military and commercial (merchant) craft Given the aforementioned ratio, hundred thousand plus warships is likely
5th edition rulebook, page 124
Black Templar Space Marines end the Catelexis Heresy by executing the Cacodominus, an alien cyborg whose formidable psychic presence allowed it to control the populace of thirteen hundred planetary systems. Alas, the Cacodominus' death scream echoes and amplifies through the Warp, burning out the minds of a billion astropaths and distorting the signal of the Astronomican. Millions upon Millions of ships are lost in the resulting upheaval and entire sub-sectors slide into barbarism without the dictats of the Adeptus Terra to guide them.
Millions upon millions of warp capable ships lost. What's notable here is that 'millions' by this is not a significant number for the Imperium to lose in a single, major catastrophe. Indeed in context its quite minor. Hundreds of thousands of warships again is quite likely from this.
Deathwatch: Rising Tempest, page 214
These regions are the domain of the Imperial Navy; made up of thousands of sector battle fleets and countless vessels the Imperial Navy patrols the void and protects the Imperium’s borders as well as the darkness between its worlds.
Thousands of battle fleets, again 100-150 thousand vessels as a reasonable approxmation.
Battlefleet Koronous: Page 67
Also called the Battlefleet Commander, lord high-admiral is the highest rank of the Imperial Navy. Only five of these august individuals exist, one for each of the Segmentae Majoris. Each is responsible for the Imperial Navy’s fighting forces across the thousands of sectors in their allotted quadrant of the galaxy, or in the case of the Lord High-Admiral Solar, the substantial volumes around Holy Terra itself.
Thousands of sectors in a single segmentum (possibly excluding Solar, depending on how you interpret) at least 8-10 thousand sectors by this, although given that some (like Ultima) are quite large this could be an under statement. Again hundreds of thousands of warships implied for hte Navy is quite possible.
Battlefleet Koronous, page 68
In ancient times when an entire Battlefleet might be massed together, the thousands of ships present would be divided in to three commands, each under their own admiral.
A rather more liberal interpetation might suggest thousands of warships, but it may include navy transports of various types as well as the warships, but hundreds of warships is reasonable from this.
Execution Hour (paperback) Page 33
Those readers less well-versed in the details of the later events of the Gothic Sector War may wonder why I have involved myself to such an extent in the events surrounding one vessel - which was, to be sure, just one of hundreds such warships amongst Battlefleet Gothic - and its so far unknown captain.
Hundreds of warships (or cruisers, if you want to be liberal) in the gothic fleet. given 'thousands' of sectors we're tlaking close to half a million warships by this interpetation.
Not even the lowliest rating or indentured slave-worker aboard any of the hundreds of Imperial Navy warships under his command would be in any doubt about the figure’s identity..
Spoken of Ravensburg for context. Confirms the 'hundreds of warships' in the gothic sector fleet. This may reflect both active warships, mothballed reserves and quite possibly other kinds of warp capable ships (Freighters, such) converted to military use.