In A New Hope we have this statement:
"The battlestation is heavily shielded, and carries a firepower greater than half the starfleet. It's defenses are designed around a direct, large-scale assault. A small, one-man fighter should be able to penetrate the outer defense."
"Firepower greater than half of starfleet" quite obviously refers to defensive emplacements - since superlaser cannot shoot fighters, it is irrelevant on fighter pilot briefing. It would be like informing interceptor sent to intercept B-17 or B-29 about amount of bombs it carries. Pilots already know it can cause massive damage to ground installations. What is important to fighter pilot is amount, type and positioning of guns it carries. Also, if Dodonna was referring to superlaser (as some suggest), following statement about heavy concentration of firepower on some segments of surface would make no sense.
According to both my and Darkstar's scalings, DSI is around 120 km in diameter. It's surface area, therefore, is 4 x pi x r^2, or 4 x pi x 60^2. That gives us surface area of 45 239 km^2. I will also ignore superlaser when calculating amount of guns.
Now we need amount of guns.
I will use these assumptions:
a) large defense towers are equal to medium turbolasers on ISD
b) small guns are equal to light turbolasers (and will not be counted)
c) 10 medium turbolasers equates 1 heavy turbolaser
d) ISD carries 16 heavy turbolasers in 8 turrets
While from first shot it is clear that large portions of Death Star have very light concentration of artillery, I will use last screenshot, which gives us 2 towers per 14400 m^2 as basis. That is 139 (138.8¨) guns per square kilometer. That times 45 239 square kilometers gives us 6 283 194 guns. That, in end, gives us 39 270 ships of ISD level equivalence.
On the other hand, large portions of Death Star are much less "populated", so I might drop that number by third, for 26 000 ships of ISD-level equivalence.
However, there is one more thing to note: 26 000 ships of ISD-level equivalence means exactly that - so, if one SSD is equal to 5 Star Destroyers, and entire fleet was composed from them, then we would have 5200 Super Star Destroyers. Also, I reduced final number by third, which, given displayed gun density is still extremely favourable for Empire - number might as well be around 12 000 ISD's without ignoring canon evidence.
Imperial Fleet size, as a wild guess, might be 15 000 ISD's and 50 000 frigates, plus 150 000 corvettes, if one frigate (same type as Rebel Medical Frigate) is 1/10th strength of an ISD, and blockade runner (corvette; such as used by Leia) 1/3rd of that - I doubt that TIE's are noticeable part of the fleet).
