At one point in like 2015 I think I was subscribed to WoW, ESO, and SWTOR, all at the same time. Outside of that one instance, though, I was only ever playing one MMO at any given time from ~2008-2018 (if I was playing one at all).
But now, other than the trial of FF14 for a week or two a while back, I haven't played any MMOs since around the time BFA came out.