You're already doing the right thing in initializing the counter:
Screen1.OnStart: Set(myCount, 0)
But you need to close and reopen the app (i.e., save it to your computer, then open it) to have the OnStart executed.
To test the value, you can use the If function in your buttons:
btnUp.OnSelect: If(myCount < 9, Set(myCount, myCount + 1))
btnDown.OnSelect: If(myCount > 0, Set(myCount, myCount - 1))
Another option is to keep the formula as is, but disable the buttons if they cannot be pressed. The properties would look something like those:
btnUp.OnSelect: Set(myCount, myCount + 1)
btnDown.OnSelect: Set(myCount, myCount - 1)
btnUp.DisplayMode: If(myCount < 9, DisplayMode.Edit, DisplayMode.Disabled)
btnDown.DisplayMode: If(myCount > 0, DisplayMode.Edit, DisplayMode.Disabled)
