What are you making your buttons with? xlabel?

Youll need some scripting module for this. There might be some nifty way to do it without mz or lua, but I doubt it.

Maybe this will work:

xTaskbarOnAdd !ScriptedBang
xTaskbarOnRemove !ScriptedBang

Then in lua:

function bang_ScriptedBang ()
if evar.xTaskbarCurrentButtonCount > xTaskbarButtonMaxCount
exec ("!LabelShow Buttons")
else
exec ("!LabelHide buttons")
end

or something...

thats just what you said, but in lua.