Just posted this issue via Windows Feedback. Please upvote it!

One other thing that was in JumpLists in 8.1 and missing in 10: ability to drag-and-drop to sort your jumplist items. I can't do that any more and I miss it terribly.

To further confuse this discussion: on both my laptop and my desktop, my max number of pinned items seems stuck at 12. If I unpin one of my currently-pinned items, one of the items I had pinned under...

Fwiw:
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Now that I've used it some more, I seem to be getting even more than 20, up to the vertical screen resolution in fact. Look.
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Yes, I'm getting 20. I was going to increase from 15 (my prior max on my laptop) but found the syncing from my desktop increased it to 20. I also found that it synced back the other way, causing the...

I'm not sure this will help, but I upgraded my laptop to Win10 and my desktop is still on Win8. I had set my Taskbar jumplists to 20 on my desktop Win8.1 machine, and the automatic syncing synced...