Quite a lot actually. They trickle in in the later levels, more so than the other readings it seems like. WaniKani likes to show you examples of all the readings pretty soon after learning the kanji, even if those readings aren’t super common, so you aren’t completely caught off guard when you see one in the wild.
The kanji 上 has a lot of possible readings, so it’s easy to be confused about how kanji readings work…
There are a lot of words using the reading you were taught with the kanji. You will see the on-yomi reading じょう for 上 in compound (jukugo) words. Two (or more) kanji compound words usually* take on-yomi readings. You will start learning words like that on WaniKani once you learnt the other kanjis that form these compounds.
Here’s a tofugu article explaining some things about kanji readings and the history
Below are some examples of 上 words using the じょう reading you will encounter on WaniKani later:
上手 (じょうず) - Good At
以上 (いじょう) - More Than
地上 (ちじょう) - Above Ground
上司 (じょうし) - Boss
Seems like the more commonly-encountered ones, that happens to, so you get to them pretty quick in WaniKani, and it makes it seem like they’re all like that. I’m only level 27, but that seems to have calmed down some.