Any idea to display the workspace number at changes?

I’d go with FvwmButtons or FvwmScript if I wanted to do this.

I don’t know how good or bad it would be, but it should work. Something in the lines of:

DestroyModuleConfig FE:*
*FE: new_page kkk

DestroyFunc kkk
AddToFunc kkk
+ I DestroyModuleConfig foofunc: *
+ I *foofunc: Geometry 300x100+111+111
+ I *foofunc: (Title 'Page: $[page.nx],$[page.ny]', Action (Mouse 1) Nop)
+ I Module FvwmButtons foofunc
+ I Wait foofunc
+ I Schedule 500 All (foofunc) Close

AddToFunc StartFunction I Module FvwmEvent FE

Untested, it should work though. That’s the basic idea with FvwmButtons.