To limit how many files you can download simultaneously (there are numerous benefits to doing so), open qBittorrent and go to Options (Alt+O or Tools -> Options).
Go to BitTorrent on the left hand panel.
Check Torrent Queueing and then input your desired values.
Torrent priority has now been enabled. You can now change the priority of torrents and limit the number of downloads and uploads to your liking. If you like, you can find out how to stop-auto seeding when a download finishes.