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

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)

dvi

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

Mini-DVI

mini-dvi

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

Micro-DVI

micro-dvi

MacBook Air (original, early 2008)

Mini-VGA

mini-vga

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

Looking at this list, I'd say that the most important adapters for you to keep around are the mini-DVI, mini-DisplayPort and the Thunderbolt ones. The other two (micro-DVI and mini-VGA) are either old and outdated or simply quite rare. So for the most part you should be safe with those initial three. If you want to be completely on the safe side though, you should look into getting those other two, older ones, as well.