I just tested in Visio 2010 and was able to add 700 Buttons to the page, no problem (ok, slow, but worked!) No code-behind, but I was able to click the buttons.
I added 800 buttons to a page in Visio 2007 (on a Virtual PC), and Visio had problems rendering the last 60 or so. You could drag-select them, but not see them!
Visio 2010 64-bit was able to handle and display 1000 buttons, although it takes around 10 seconds just to toggle between run-mode and design-mode.