How to send 1 mln requests asyncronously - mechanism?

One needs to send 1 mln HTTP requests concurrently, in batches, and read the responses. No more than 100 requests at a time.

Which way will it be better, recommended, idiomatic?

• Send 100 ones, wait for them to finish, send another 100, wait for them to finish… and so on

• Send 100 ones. As a a request among the 100 finishes, add a new one into the pool. “Done - add a new one. Done - add a new one”. As a stream.