how does zaroff finds men to hunt ?

The island is called "Ship Trap Island". Zaroff uses a lighthouse to trick ships into running ashore. He traps the men and hunts them.

so the sound of the gunshoot was deliberate or intended , and also killing the buffalo , zaroff now is playing adangerous game , so what is the benefit of killing abuffalo , can you describe ?? is it intended ?

No, Zaroff was hunting a person. The gunshot he hears from the island was Zaroff hunting and killing a person. Zaroff stated that he had stopped hunting animals a long time ago.

he (rainsford ) stated that it is abuffalo !