... like two at a time... maaaaaybe three, but honestly, I prefer to just focus on watching one show at a time these days (when I was keeping up with Pokemon, Pokemon was always there, while something else was something I'd watch on the side, almost invariably a short/completed series, since long-runners, I can't commit to much anymore! When I did watch long-runners, it was REALLY long ones like Pokemon, Detective Conan, etc.).
And I don't marathon episodes! At MOST, I'll see two maaaaaaaaaaaaaaaaybe three episodes a day, but usually, I can only handle one or two. I won't remember what happened if I watch too many! ^^;;;
Ironically, when I'm reading, it's rather different, and I'll read like 3 or 4 books at once. Manga is faster (I can read at my own pace and cast and score the "show" in my head however I like!) and more discrete, so I prefer that these days, it's also usually more beautiful to look at (with some exceptions), given more time to spend on the fewer frames (panels, really), compared to animation, so it's more inspiring to me as an artist too!