Does anyone know how much PS the reservoir holds?

If you have your owner manual chances are it will be listed as fluid type & specifications.

My guess less than 1/2 of a quart, but only with a syringe with hose will you actually be able to get most out. It seems there is always still some at bottom of reservoir.

Did you find any air bubbles in the reservoir after running pump wheels back & forth?

No air bubbles. I changed the fluid last night. 18oz later, I still have a whine

Did it quieten down at all? It will have a bit of a whine when at full turns left/right is normal.
If it is screaming all the time..might want to try a new pump!
I have also found that on noisy pumps sometimes adding this product will reduce friction & get it a bit quieter...I generally mix Lubegard 50-50 with normal fluid.
LUBEGARD Power Steering Fluid

No, it didn't quiet down. Hopefully here in a couple days it will when it runs some clean fluid through it

It is quieter now. Just gets higher and louder under acceleration

You could let it run with new fluid for a few hundred miles, then suction it out again.
I used a mix of lubegard and GM Power steering fluid on my 2nd go around and it was much better on my PA noisy pump then.