Can anyone tell me how the number of Torrent seeders depends on the download rate? Or at least that's what I've heard from some of my friends.

The download rate depends on the number of seeders.
More seeders = More download speed

When you get packets from more seeders, you download more for once, while on HTTP, you can only use one stream, when not using Download Accelerators.

This is true if your ISP doesn't limit your max speed, though.

it's all down to the seeders upload speed
you can have just one seeder and get a very good speed if the seeder has good upload
yet you can have lots of seeders with a slow upload speed and get a slow download
numbers are better most of the time