I finished the year with 19,926 squares (and could've bumped that up to a round 20,000 if I'd really tried), but the square stalled at 39x39 after I shagged my knee, and I've been a bit reluctant to go hike-a-biking, which is kinda what I need to do to get the squares to push it up to 40. The Pennines is tough terrain.
I should make progress on my cluster in 2020 (currently 3757), and I've a couple of well-covered areas around Berwick and the Lakes where I just need to get a few key* tiles to link it all together, then my cluster would be coast-to-coast from the North Sea to the Irish Sea, and from the Humber to the Tweed.
*read: fucking awkward. I had planned to get them riding back from the Lakes in April, but that's when I shagged my knee.