Prior to the 1.13 update, Explorer Maps were numbered, and you could see this number when you hovered over them in Inventory, or even the Trading screen. I remember seeing Woodland Mansion maps #50 and #99, and at least two others.
Just prior to the 1.13 update, I traded for map #99 at a distant and newly discovered village, and viewed it. I was planning on collecting the different ones I'd seen up for trade before.
When I signed in after 1.13 I noticed the number had vanished from the name of the new Woodland Explorer Map. I returned to a very familiar village where I'd known a Cartographer had been offering #50 for quite some time. Hovering over the map in the Trading screen showed no number. I traded for it and it still lacked a number, and was in fact identical to the map I had previously known as #99.