Do you by chance have any monitoring set up on this server like PRTG that might track the thread counts over time? These are super helpful for tracking these types of things down. Absent that though, we could do a review of your logs and get a trace or two to look for any signs of trouble.

Seeing 100+ here all the time.

On the test servers I do most of the management for I see 90-120 depending on what's going on. Today I've had some users synchronizing larger mailboxes so these might be higher than usual currently. On another test server I see north of 350, but this server has a significantly higher user count on various protocols. It'll vary, so if you're seeing an increase I'd be on the lookout for any recently onboarded clients, users using new protocols, heck even a new device could introduce a thread increase with iOS devices (in particular) utilizing multiple simultaneous connections to synchronize account data quickly. 

That said, it sounds like David is seeing this increase after just an upgrade so I wonder if there might be another explanation for it in his case. If you need any help looking into things @David we'd be happy to help out on a ticket.