Answer 1

solo faq:
https://www.sololearn.com/faq

https://code.sololearn.com/Wm08eYGJw1hc/?ref=app

Answer 2

21 seems to be the max as of now.

Similar questions.
https://www.sololearn.com/discuss/476823/?ref=app
https://www.sololearn.com/discuss/160312/?ref=app
https://www.sololearn.com/discuss/404006/?ref=app
https://www.sololearn.com/discuss/1051863/?ref=app
https://www.sololearn.com/discuss/1790746/?ref=app

Answer 3

Reaching level 20 is a great milestone.
Sololearn bot is at level 21

Answer 4

Every level above 16 is completely useless, you can only get there by doing the same challenges over and over and over and over again. (And I missed a few overs).