Two qts, and at either the Allen wrench plug under the slave cylinder, if you can get it out (I cannot in my 91S), or the reverse gear back up light sensor located further to the left (car left) and back at the top of the gear case (this location is what I use). I use a long skinny hose and a funnel.
Is the 75W-90 the NS or regular? Richard Jemison strongly recommend using the NS version. I do use it in my 94LS to quiet the gear noise down a little (it's a 1995 Q transmission, and the the gears, esp 3rd, were a little noisy on partial throttle), and it seems to work just fine (so far).
My 91S has Royal Purple ATF in it, which works ok, but don't know how it will work when the temps drop low next winter. When I had the clutch replaced in that car a couple of years ago, Carlo put in Kendall ATF (as he usually does I guess), but the transmission was pretty darn balky and stiff when the temperature was in the 30s and 40s until the fluid warmed a little. Didn't like that, and don't remember it being that way before.
I had been running the Redline 75W-90NS for a short while before the clutch change, and when he asked me what was in the transmission, and I told him, he was not all that happy with the gear oil, telling me that Alfa had said not to use gear oil because it was too stiff when cold, not getting to all the bearings quickly enough. I mentioned that the rest of the world seemed to use gear oil, and he didn't say anything. We changed the subject. So... I do not know.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6