It's been a while since I was on Win8 so bear with me, but the Win10 taskmgr is pretty much the same so I am pretty confident this is the answer you are after.
The column you are looking for is no longer on the "Processes" tab of the Task Manager window like you are used to. You now have to go to the Details tab, right click in the column headers, and choose "Select columns" from the dropdown menu. From there you would put a checkmark in the Threads row to see the threads in the Details grid.