MySQL AB conservatively estimates 4,000,000 installations worldwide. With 10,000,000 downloads from mysql.com annually, and countless other distribution agreements (e.g. bundling with practically every Linux distribution, custom RPMs, etc.), that number could easily be off by an order of magnitude. The truth is no one knows how many MySQL installations are out there, but 4,000,000 is a rough lower bound.
MySQL AB also claims roughly 4,000 paying customers (licenses, support, license+support). That's about as big as Sybase's customer list. :-)
