Personal: right now I have 6 open. I might get up to 15-20 if I'm going down a rabbithole of some sort.
On a side note, Windows 11 finally put in an option to go back to normal Windows taskbar buttons so I can actually read tab titles from the taskbar:
Work (software engineer): sometimes dozens if I'm deep in the weeds with loads of reference pages open/etc.
Just used one today to finish listening to a YouTube video while I was in the shower. It's not a "bathroom" speaker per se, just a pretty nice Bluetooth one, Anker Soundcore Boost, with enough volume to rise above the white noise of the shower and ventilation fan.