Computer fan stir plate- how many magnets?

I'm in the process of finishing my computer fan stir plates and I've read how some of you prefer one magnet and others two. I have four magnets that I took from two hard drives ( I'm building two stir plates). One of the stir plates will be built from a square cigar box made of cedar. The other will be made using a plastic project box from radio shack. The fans are standard computer fans. Thanks in advance.

Chris

I have a cigar box / comp fan setup and I use one hdd magnet... Can spin a 2L starter with no problem...

I used 3 HD magnets on the first 2 stir plates I built. That would spin a 1/2" stir bar perfectly, but not a 2" bar.

Tupperware box with 1 HDD magnet spins my 1 and 2 inch bars. The only prob is the tupperware flexes and a full 2l flask bottoms out occasionally. Needs more fine tuning.