There are over 3,500 skills available on Khan Academy, as of 2020. However, this number would've surely increased, so I'd comfortably say that there are around 4,000 skills on Khan Academy as of now. However, I'm pretty sure that there isn't an indicator on how many skills there are, on Khan Academy. 

However, if you were looking at a course (for example Algebra I), which has a total of 18,200 total possible mastery points. Divide that by 100, and you'll get that their are a total of 182 skills you can master throughout the whole course. When you master a skill, you'll get 100 points, but if you get proficient in a skill, you'll get 80/100 possible points.

The new Black Hole badge requires you to master 5,050 skills in order to get it (reach Level 100). Seeing as this badge isn't actually concealed as ??? like the others are, I'd assume it's possible.

ArthurZ That also means Atlas is possible (if the requirement stayed at 5000 mastered skills)... wow!

I wonder if the Artemis badge (7,500) is also achievable by now...

I like greek myths too! @Daughter of Hephaestus

The Maelstorm badge requires 4950 skill (level 100) ! If there is only about 4000 skills, then how can you achieve the badge?! IT'S IMPOSSIBLE! And above of all, who masters that much skills?

I'm pretty sure that there are over 5,000 skills by now(at the very least) as Shall Win's estimate is from around 4-5 years ago

is there like 5000 skills including science, english, etc?

Yes, I think