HOW MANY ARs???
That lies 100% on YOU.
Are you the Minimalist?
If so you can get by nicely on ONE. The rifle is modular, allowing you to make anything you can think of with a single lower receiver, from a 7.5" SBR to a 24" plinker; from 22lr to .50 Beowulf. If you don't mind swapping out Uppers & buttstocks all you need is one lower, hence one AR 15 & you can have whatever you want.
Tactical Practical?
Own a convenient store in LA? Sucks to be you!!! But if you believe in home defense or extended security you may want to consider more than one, and fewer than 10. This allows for a rifle for you and all "shooters" that would be defending your home/business from looters in a riot situation or any other terrible ocurance that hopefully would never happen. If you have a good friend and an assertive girlfriend then 3 (min.) would be a number to shoot for. Have a big family? maybe 8 or more.
Lazy and lusting?
You want an AR in configuration X,Y, & Z, but you don't want to fool with a buttstock wrench. Plus the extras come in handy in a SHTF situation. you buy or build up the complete rifles in the configuration(s) you desire. If you were to lust after an AR in every caliber that would be at least 10 ARs right there. If you wanted a couple calibers with a couple different upper lengths you could make 24 easily without having to strain your brain for creativity.
Collector?
The wonderful thing about being a collector is that you have a relic that commands awe and respect for the rich heritage of ARs/M16s. The shitty thing is that every/any bullet fired destroys its collector's value, which contradicts the title. Therefore many collectors have "safe queens" and "shooters". Now you have to ask how many ARs is enough to complete my collection and how many is enough to round out my shooting needs? that could range from 3 to 300
The answer then is ONE rifle minimum + whatever additional desires or purposes you may have in mind.