I can math this.
Galaxy = approximately 1,024 regional sectors grouped into 20 Oversectors
Each Sector contains a fleet of approximately 2,400 ships, with 24 being ISDs and 1,600 being other combat vessels, while the rest would be support ships
This is why the Empire is said to have approximately 25,000 Star Destroyers, since 24x1024=24,576
All sector fleets combined would be 2,400x1,024=2,457,600 vessels in sector fleets alone, 1,638,400 of those being combat vessels that are not ISDs
A Sector Fleet is divided into multiple Fleets, which are in turn divided into Systems Forces, then into Squadrons, then into Lines
An average Fleet might consist of about 6 ISDs with 400-500 supporting vessels of other types.
Obviously the Imperial navy gets a lot more complicated than this because of SSDs, individual unique vessels, creation of fleets for other purposes aside from Sector grouping, like Darth Vader's Death Squadron, experimental vessels, and the ability of Moffs and high ranking admirals to appropriate their own militias and forces, usually making use of outdated vessels in the outer rim, like Venators and Acclamators, and so on.
Other Fleets, example, Death Squadron: 1 SSD and 5 ISDs with the ability to appropriate vessels from other fleets, so that at Endor it consisted of the Executor, 2 battlecruisers, 3 Tectors and 33 ISDs. This fluidity to fleet composition makes nailing down the size of the Imperial Navy almost impossible.
Each Sector was intended to have an SSD to serve as a command ship, but this was never seen to fruition, with less than 20 known Executor-Class being built along with a handful of Mandators, Assertors, Vengeances and the Eclipse. The Bellator was almost small enough to fall into the battlecruiser range, and they seem to have acted as command ships and/or novelty vessels, taking the place of ISDs whenever one was available.
But lets just say that the number of SSDs represents an approximate number of roaming/mission fleets for arguments sake. Thats about 25 additional fleets, each with that 6 ISDs and 400-500 other vessels. = 12,650.
Total Imperial Naval strength (low-balling) = 2,470,250 vessels
Total ISDs (Average) = 24,726