They would rapidly each settle on

18

I ran a simulation and that number wins against any other number.
In some cases it is not the optimal for example against 16 you should play 17.

They would rapidly each settle on

18

I ran a simulation and that number wins against any other number.
In some cases it is not the optimal for example against 15, 16 and 19 you should play

17

You have a play that is not exploitable so you can just plain solve it.