I’ve had this issue for a while now where I can only track 2 achievements at once (by shift+left clicking on the achievement within the achievements window, as if it’s an actual quest) and when I try to track more it prompts with red letters: “You may only track 10 achievements at a time.” - I don’t know how to fix this issue, as I do like tracking achievements to keep an eye on them and be reminded that I’m working on them.
(In case the question will arrise if I actually have 10 achievements tracked but they are pushed away by quests: I’ve already checked that and removed all the achievements I had been tracking, but the limit remains 2 for me.)
I’ve come across this before. It happens when you have had achievements on the tracker that you complete. When completed they go from the tracker but don’t stop tracking (remain ticked) in the achievement panel.
So basically, somewhere you have 8 completed achievements still ticked somewhere and you’ll have to track them down and untick them.
Alternative is to try deleting your WTF and cache folders.
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So I decided to delete my WTF and cache folders as finding what I tracked before is going to take absolutely forever. Either way, I lost a lot of addon settings and weak aura’s. Which I evidently knew that was going to happen, however… it did not solve the issue. 
Edit: I decided to look systematically through all my achievements that I’ve completed and was able to find 7 still ticked. Not sure where the last one is, but I’m now able to track 9, which is a lot better. I will find that one sneaky achievement some day… Thanks for the help!