This all depends on your storage, personal preference.
Two things:
- Usually, hard drives like to write only one thing.
- The faster you download, the less time your hard drive must play catch up.
So, if you have FAST speeds and download well-seeded torrents, set a low limit.
If you only have like... 10-50mbps, you can use 5 even.
I usually set them to:
Max active download: 3
Max active upload: unlimited
Max active torrents: unlimited
(To set unlimited, go to 0 and then down arrow, and it shows infinite.)
I also use the "Do not count slow torrents" and set like 1000kbps or something.
So if I add like a bunch of not-so-well-seeded torrents, they will just all start downloading.