There are two "Pokemon Showcase" pokestops within ten minutes walk from me (and literally 100 regular pokestops). Every weekend and every Mon-Wed a Pokemon Showcase will have a new challenge "show off your biggest/smallest <specific pokemon species>". People submit a pokemon and the winner gets some XP or coins or something and other contestants get a bit less based on their rank.
These two stops regularly have 200 pokemon submitted. That's what I'm basing my 1000 players estimate on. It's a low estimate. I play every day and only once submitted a pokemon to the showcase.
Ten thousand people live within my housing estate, three minutes walk from each other. It is merely one of many estates in the area.