You probably need to lower the winners.
Maybe only have one winner.
Higher prizes means more engagement.
1st, 2nd, 3rd is a good option.

You could always SHA-256 hash a password to verify you posted the number before the winners were announced.

https://emn178.github.io/online-tools/sha256.html

For example, if you SHA-256:

the-number-i-picked-was-1337

You get back:
1aed310949b8bf9a5115c5a7bf38457bff1a0db6dedc943e9739a3cefd0a37e7

You could then post 1aed310949b8bf9a5115c5a7bf38457bff1a0db6dedc943e9739a3cefd0a37e7 to the blockchain and prove what your number was after the fact. (in this case 1337)