Curious as to why I only got 12 Invisalign plates?

Hi, Adult male of 32 years of age. I used to have braces as a teenager but was not a committed retainer wearer and had some teeth revert to their crooked state due to crowding (especially the Central incisor, and lateral incisors of my upper teeth). When doing research into Invaslign i noticed many patients who required 50+ plates whose teeth were seemingly a lot straighter than mine. I received only (to my pleasant surprise) only 12 plates. How can this be?