what are some good online tutorial for learning the concepts of number theory?

If you really wanna practice for number theory from Easy to Hard level then hackerrank is best which contain all the questions and will clear your whole concept in Number Theory. Read Here

You can also prefer Hackerearth and codechef Tutorials and practice there. Hackearth Codechef

If you wanna practice more then you can prefer a2oj.com

Adding to what bansal said-

Hackerrank is best for solving problems first. Then you can advance to others like hackerearth etc.

In case, you need to learn some concepts on it, here are some answers on how to learn number theory [w.r.t. Competitive Coding]

I am glad I was able to help you dear!!

Adding to the above comments, You can also practice the problems on Project Euler. This website contains lot of mathematical problems and is very good website for number theory problems.

Happy Coding!

Yes and here is a link to Mimino’s repository on Project Euler problems … He solved about 77 problems. You can use them for reference if you get stuck on Project Euler.