Kahoot! has a max limit of 4,000 players joining the same game PIN. However, kahoots of this magnitude require excellent bandwidth, WiFi, and/or cellular reception.
Also, please let us know if you are planning a kahoot of 1000+ participants and we can advise you on the best course of action. To reach us, please login to your account and use the "Support" button in the top nav bar.
For details on how a kahoot is hosted and/or played, check out this article.