I went looking on Answers.com for the reasoning behind why 999 is titled 9 Hours, 9 Persons (not People), 9 Doors.
I got a decent answer!
- Persons refer to several different individuals, considered separately.
- People refers to a group of individuals with a similar interest.
Since 999 deals with 9 individuals who seem to have nothing in common, they're considered persons rather than people.
However, I wanted to point you all to the results surrounding the Answers.com website that people seem to ask. I got a decent answer out of Answers.com but it seems there are some odd Questions:

I don't even want to know what a loaf of Brad costs today!
I got a decent answer!
- Persons refer to several different individuals, considered separately.
- People refers to a group of individuals with a similar interest.
Since 999 deals with 9 individuals who seem to have nothing in common, they're considered persons rather than people.
However, I wanted to point you all to the results surrounding the Answers.com website that people seem to ask. I got a decent answer out of Answers.com but it seems there are some odd Questions:

I don't even want to know what a loaf of Brad costs today!
no subject