Steam trading cards question

All of them. However, your drop rate is reduced significantly.
(i.e. I left 4 card games running at the same time overnight. When I woke up 8 hours later. None of the games had dropped all the cards.)

Why not simply use idle master ? Just launch it and let it do the job. It will automatically pretend to launch each game, then periodically check for card drops and then move onto the next game.

only one
use idlemaster for the best card collecting