How to handle 500 HTTP req/s to 3k servers as a client in Spring Boot?

I'm creating an application that must send about 500 HTTP notifications per second. It's not so big number of requests, but there is a lot of application that I must connect to, so keeping connection open (keep alive or sth) is impossible. Is there a better way than starts 200 threads and opening new http connection each time? Is Reactive programming suitable in this case?