Does each language have as much (or about as much) skills?
No. Last time I checked it was from about 30 to about 120 skills, with Esperanto, High Valyrian, and Turkish at the low end, and Norwegian and German at the high end. The number of lessons varies from about 300 to about 600 (I suspect 600 is an underestimate). Someone with a script for summing them could give you precise figures.
Japanese and Korean are also at the low end. I think Japanese is the smallest course so far.
Why is that do you know? I understand that high valerian and Esperanto skills could be limited as they are "made up" languages designed by linguists, but what about the other ones ? Did the contributors who, very generously, worked for free just run out of steam?
No that's simply because those languages were released very recently. For example japanese was released a few months ago while korean only 2 weeks ago. They're still in their Beta phase and need quite a bit of work, especially japanese.