This is because six people is the perfect number when smoking weed as any more will result in overcrowding of the tiny smoking shack and any less will result in the group becoming bored.
Bill: Fuck that let's get high with the sexy six
John: Let's invite Manuel to smoke with us!
Alex: No way my shack is too small and shitty to accommodate more people
SEXY SIX BITCHHHHH <3