How many connections your client can handle will depend both on your computer's RAM/CPU and the bandwidth of your connection. Each connection uses a certain amount of your PC's resources as overhead, and the same with your bandwidth; you can configure many clients to display this overhead cost with the up/down speeds.
What's counterintuitive is that - up to a point! - more connections generally = more overall speed. Think of it this way: you're "packing" data more efficiently into each data packet, while only one (or just a few) connections might not completely fill each individual packet. The overhead
will eventually diminish those returns, however, and furthermore, vast numbers of connections
will bog down any PC, so it's definitely not magic, mind you!
My connection is roughly 22 Mbit down/4 Mbit up, and my PC runs a Core2Quad Q9650 ("Yorkfield") @ 3.0 GHz, with 8 GB RAM. My current connection limits are: 500 connections total, 500 connections per torrent (in other words, if 1 torrent is active, it can use *all* the connections), and # of upload slots is set to 4, with "Add additional upload slots if speed <90%" checked; downloads are unlimited, and uploads are capped at 2 Mbit. Setting the max per-torrent to the same as total max connections allows individual torrents to grab more connections, if fewer are active (not just upload slots); uTorrent is very good at NOT inappropriately applying all of them to only one torrent, don't worry ^^' ... Currently, I have 87 torrents in "Forced Seeding", with 10-30 active at any given time seeding at ~75-250 KByte/s. This is all using uTorrent 2.2.1 on Win7 x64 Ultimate. YMMV depending on your PC's specs, available bandwidth, and OS, of course.
Anyway, at those settings, I see absolutely *no* bogging of my PC. Even when simultaneously playing relatively heavy-duty games, such as Crysis 2, I don't find much, if any, performance drop. If it's d/ling at max (~3 MByte/s), I DO see slower net, but only then. I can run 1000+ connections with no issues, but I don't feel much need to

.
The 80% rule, btw, is a good one for your upload limit. Any more, and you'll likely bottleneck the "ACK" packets the clients - and in fact, just about every net-using app - send(s) after receiving data packets. If those are held up, the client sending the data waits until each one comse in. slowing down your overall connection a
lot; it's a normal limitation of TCP/IP. Limiting download bandwidth is usually not necessary, unless you need to reserve some for another use (an online game, perhaps).