this is second time, I am in Torment 12, zero people fighting world boss. Anyone know why?

go to character select and rejoin instance, empty lobby it happens if there was a recent difficulty change or other conditions

Maybe silly but double check that you have crossplay enabled.

Valid

Both post #2 and #3 are good advice.

Also suggest checking for social tab - group finder WB. Or making your own lobby 5 mins before. If you’re showing up at the world boss AFTER he’s spawned everyone’s gone and he’s dead instantly this season.