random user selection

Hello, Im trying to build a website, that will choose a random user from my list of users,

I want it to be tiggerd by the number of people such as “100”, I want the raffle to start when I get “100”..Not by a specific date

I found plugins that have drawing or sweepsteaks, but they all are triggerd on the date, and not by the amount of people

I would like it to be automatic if at all possible.

Any suggestions would be truly appreciated.
Thanks everybody