Although how many ships either side of the Galactic Civil War had is never precisely specified in the movies, there are a few hints. At the time of SW4, Han Solo indirectly implied that the Imperial Starfleet consisted of significantly less than "a thousand ships" - which coincides with his surprise at the presence of an Imperial Cruiser (the small "Star Destroyer" seen in SW4) at Tatooine (an indication that the Imperial Starfleet had fewer such vessels than the Empire had full-fledged member planets - 1,024 according to the Senate chamber scene in SW1 - and that the "bulk cruisers" used by system authorities as patrol ships are not considered part of the Imperial Starfleet, since there are presumably an average of several such ships per member system). Given that the first Death Star is stated to have a firepower "greater than half the Starfleet" (which IMHO excludes the superlaser to avoid conflict with Solo's earlier statement that destroying a planet was beyond the capability of the entire Imperial Starfleet), this places an upper limit of 445 Star Cruisers at the time of the Battle of Yavin (with the surface defenses of the first Death Star representing a raw firepower almost 56% of an even 400 Star Cruisers). By the time of SW5, Imperial naval shipbuilding had expanded enormously (no doubt as a result of the Imperial Senate's dissolving in SW4), with both the larger Imperial Star Destroyer and the gargantuan Super Star Destroyer in full production. Official sources tend to agree that there were 4 Super Star Destroyers in service at the time of SW5, and Han Solo's comment in "Return of the Jedi" of there being "a lot of command ships" gives weight to Curtis Saxton's guesstimate of 10 Super Star Destroyers having been built by the time of the Battle of Endor (likewise, there were doubtless several other Super Star Destroyers present at the Battle of Endor, as indicated by Admiral Ackbar's order to "Concentrate all fire on that Super Star Destroyer" as opposed to "the Super Star Destroyer"). Assuming that the Galactic Empire was capable of launching (on average) a Super Star Destroyer per month, that warship production is in inverse proportion to mass1/3 (which warship firepower and hull strength in the Star Wars galaxy is directly proportional to - as shown by how larger ships have proportionally smaller main armament and weaker hulls), and that wartime production of the ISD and ISC began exactly one and two years before that of the SSD (perhaps coinciding with some benchmark in the Imperial Fiscal Year), this implies a production of 80 ISDs and 700 ISCs between SW4 and SW5, and an additional 30 ISDs and 150 ISCs built by the end of SW6 almost 7 months later (with some 10 ISDs and 250 ISCs lost to various causes, primarily combat, between SW4 and SW6).
Naturally, the Rebel Alliance (with only the outright support of a handful of systems) had far fewer ships (even if their military strength had grown even more enormously from the 22 X-Wing and 8 Y-Wing fighters we saw in SW4). At Endor (which, even more the Battle of Yavin, was a "kitchen sink" operation, and so involved every last ship - even the cargo transports - of the Alliance fleet), up to nine Mon Calamari Star Cruisers of three types were observed - of which two were of the largest Home One type (IMHO, the second cruiser destroyed by the Death Star superlaser in SW6 is not a Home One type, due to the asymmetrically large starboard dorsal bulge and lack of dorsal towers - which, as the hyperspace exit scene showed, is a characteristic of Liberty-type vessels instead of Home Ones). This establishes that, at least among Rebel warship subgroups, Rebel warship representation is likewise roughly inversely proportional to mass1/3. The Rebel Fleet's hyperspace jump in SW6 gives a rough indication of the fleet's overall makeup - accompanying the lone Calamari Star Cruiser actually observed jumping were two Nebulon-B escort frigates, three heavy transports, and four Corellian corvettes. At least two Dreadnaught-type vessels (probably the canonical "Rebel assault frigates") were seen during the Millennium Falcon's flyby of Home One just prior to the jump, and what has become known as the "Y-Head Corvette" (which IMHO is likely to be a modified Dreadnaught viewed from a different angle) was also seen in the second fleet battle sequence. Blockade runners (the third-sized corvette seen at the beginning of SW4) were also present (witness the tiny "corvette" in front of the Nebulon-B during the latter's exchange of fire with an SSD just before the Endor shield was brought down) - presumably, three were present for every corvette, if the inverse mass1/3 ratio holds.
It should be emphasized that these numbers represent the peak of the Star Wars galaxy's military might - as the galaxy-wide celebratory ending sequences of the SW6 Special Edition strongly implied (no, I generally do not take the Expanded Universe material seriously), never before or again would that galaxy ever witness such numbers (or military tonnage). At Endor, the Alliance lost 20% of its forces in that galaxy's Armageddon (and would demobilize many more in the days afterward), while the Empire lost the second Death Star and practically the entire Imperial Navy. (Those who doubt the ability of the Rebels to triumph over vastly superior Imperial forces need only look at the Battle of Yavin - in which, at most, 15 X-Wings and 4 Y-Wings were lost in dogfights that claimed almost all of the Death Star's 14,400 TIE Fighters. No doubt a significant factor in this lopsided kill-loss ratio is the legendary inaccuracy of Imperial gunnery, even in stationary point-blank capital ship combat.) Given the grand, happy ending that George Lucas provided for his mythic saga in SW6:SE, IMHO the few Imperial starships that were not present at Endor were captured during the galaxy-wide uprising seen at the end of SW6:SE and sent to the shipbreakers as part of a purge of the major symbols of the evil Galactic Empire - just like the toppled statues of the Emperor on Coruscant itself.