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][1]
You can also prefer Hackerearth ~~Tutorial ~~and codechef Tutorials and practice ~~ther. [Click][2]
~~there. [Hackearth][2] [Codechef][3]
If you wanna practice more then you can prefer ~~[a2oj.com][3] ~~[a2oj.com][4]
[1]: https://www.hackerrank.com/domains/mathematics/number-theory/page:1
[2]: https://www.hackerearth.com/practice/math/number-theory/basic-number-theory-1/tutorial/
[3]: https://www.codechef.com/wiki/tutorial-number-theory
[4]: https://a2oj.com/category?ID=41