Allegiance to a dynasty was once purely a matter of family and tradition, but it is now entrenched through conquest and programming. Before the coming of the C'tan, there were many hundreds of Necrontyr dynasties. Through the Wars of Secession, the War in Heaven, and the rebellion against the C'tan, many of these dynasties were destroyed. It is impossible to say how many survived, though the number is likely in the hundreds.
Since the Silent King destroyed the protocols of control over the Necron race, the Necron Dynasties now act independently from one another and pursue their own agendas. Each Dynasty is ruled over by a Necron Overlord while the most powerful Dynasties, ones which encompass an entire Sector, may be ruled over by Phaerons. Currently, the Sautekh Dynasty is the most powerful of the Necron race.