I have not stumbled into a limit to the number of ephones that can have the same ephone-dn assigned as a shared line button.
I went looking for a limit and couldnt find one.
I found some CME blogs talking about 100 as a tested case.
In my lab I have a UC500 I with 23 phones (all different types that have more than 1 button) with button two being a shared line and it works.
Steve