We did some rug beating on this topic a few weeks ago. Look in the older topics for the title "Experience before advanced ow"
. Also check a similar discussion on the ScubaBoard.com forums.
Essentially, becoming an Advanced diver is something of a personal thing. While many of the popular courses (PADI, NAUI, and others) have classes called "Advanced Open Water", my opinion is that taking the class does not automtically make you an "Advanced" diver like the first class you took making you an "Open Water" diver.
Taking the class is a good idea, as is always taking a class towards the next level or specialization, but it's up to you whether or not you can truly call yourself an advanced diver.
As far as number of dives before you can take the class, it depends on the training agency. PADI & NAUI will allow you to take the class directly after your 4 open water basic dives. Others might make you wait until you have "x" number of dives.