mathdash is the closest platform.

You should visit this website here two types of contest are held one contest is where only the answer is required for question which happens weekly mostly targetting a younger audience

and biweeky (after every two weeks) a solid proof based contest is held which is graded live by the creators of the web here is the link

Roughly, if you are looking for just Competitive Mathematics related Duels, there's a website called http://www.matiks.com/. It has mobile application too and gamifies mathematics. Try it to see if this is what you were looking for.

Unfortunately, nothing like CF exists in math because rating mathematical proofs is quite difficult, since unlike code, math proofs are not always built in some unified formal language. If you just want to improve your number theory skills it seems that solving past school olympiads like BMO/IZhO/RMM etc or books like "NUMBER THEORY Structures, Examples, and Problems" by Titu Andreescu is better, Well it is better if you look for smth similar to cp problems, if you looking for some uni level research problem then idk

In this blog [Tutorial] A way to Practice Competitive Programmig by E869120 writed:

Sometimes, especially in Div1+Div2 round, some problems need mathematical concepts or thinking. Since there are a lot of problems which uses them (and also light-implementation!) in TopCoder, you should solve TopCoder problems.

So maybe TopCoder Div1Easy good place to improve math. (I haven't tried it yet)

Try Project Euler

practice on AOPS ,it has got nice problems ;)