The NAR Model Rocket Sporting Code or "Pink Book" limits competition models to three stages. I "thought" it was in the Safety Code, but that might have been eliminated to simplify it. Vern Estes and the other early manufacturers supported the NAR, so they adhered to the Safety Code.
The Pink Book was named as when it was first printed there was pink paper available for the cover, it was apparently the cheapest, and the rest is histerical (pun intended)

I have never heard of any serious manufacuters offering any models with more that three stages. It does get to a reliability issue with the "average modeler", but I am sure it could be done by anyone wanted to go that far. Just not with a NAR launch or insurance. A good exclusionary case is the "rack rocket", but that's another show.

Chas

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Charles Russell, MSgt,USAF (ret.)
NAR 9790, Lvl 1
SAM "Balls Three"