Just to get it out of the way, the cold lines are only for reference doing an initial fill. The level you actually care about is going to be the hot level, and to check that you want the car on level ground at operating temperature with the engine running. Run the gear shift from park down through all the gears and then back to park, then check the fluid level.

As far as how much fluid you can safely add at one time, there isn't really an answer to that. You can safely add as much as you need to bring the fluid level to the correct range. Exactly how much that is depends on how low you are. I'd go with smaller increments and repeated checks to avoid overfilling.

If you were already up to the cold low level, 2 quarts should have moved the level on the dipstick a solid amount. Just to check, where are you adding the transmission fluid?