Just one, unless on the following two occasions.
1) When I'm choosing my next badge - my badge list and the official badge list (this is so I can color coordinate - idk if I'm the only one who does this, it could be my OCD tendences, idk... but I always look for not only the badge of somebody I stan, but also for one with a color scheme that matches my current layout)
2) When I'm in the process of trading cards - the thread of the person I want to trade with, their cards list (if necessary), my trading thread, and my cards list (so I can make sure I don't accidentally offer a card I don't have or miss an opportunity to snatch one I want)