How many different VGA adapters for Macbooks exist?
With the help of Mactracker, I've compiled a list of all the connectors used on Mac portables in the last decade or so.

If you stick with machines since 2009 (actually late 2008 discounting one iteration of the 17" MBP), you can get away with just a mini-Displayport adapter.

Adding to that a DVI and mini-DVI adapter will cover you every Intel (2006 and on) portable (as well as most G4 PowerBooks), except for the first MacBook Air (of which there aren't many, but it requires a micro-DVI adapter).

Mini-Displayport/Thunderbolt




MacBook Pro 13" and 15": Late 2008 and on


17": Early 2009 and on

MacBook: Late 2009 and Mid 2010
MacBook (Aluminum): Late 2008
MacBook Air: Late 2008 and on


DVI (full-size)




MacBook Pro: 2006 to early 2008; late 2008 (17" only)
PowerBook G4 (15" and 17"): April 2002 and on


Mini-DVI




MacBook: early 2006 to Mid 2009
PowerBook G4 (12"): September 2003 and on


Micro-DVI




MacBook Air (original, early 2008)


Mini-VGA




iBook: May 2002 and on
PowerBook G4 (12"): January 2003 model (PowerBook6,1)