It's OK. Took me a long time to sort it out, too.
My humble opinion is:
Lots is good. More is better. Too much is just enough. :]
What would be more helpful, though, is an idea how to maximize the money spent on magnets when they're so expensive. Get the most bang for the buck.
I've never used magnets thicker than 1/2" for example. Using thicker magnets would increase the flux, but my guess is only slightly, and the practical problem of fitting a rectangle on the outside of a cylinder makes thicker and thicker magnets a losing battle.
The other dimensions left, the length and circumference of the rotor, are pretty good for you, since the rotors on older motors was much bigger than they are now. I just did a sketch of your 5.5 diameter rotor. I can only fit 27 1/2" x 1/2" magnets around the circumference, not 28 like was said before. Spacing out 24 magnets equally around the circumference fits better. Too bad because 28 would cog much less than 24.
The rotor looks long enough to fit 3" of magnets, so I'd look at fitting 3 rings of them around, staggered a little to reduce cogging.
Back to KJ Magnetics... 1" long x 1/2" wide x 1/2" thick... Uhhhkk sorry 500 bucks.
So this is the point where we get motivated to make it more affordable!
What would happen if you used cheaper 1/4" thick magnets instead? The funny ones I found earlier are much cheaper, and because they're thinner you can go back to fitting 28 around the circumference. 90 mags x 3.77$ is about 340 dollars. Their website knows I'm in Canada, so it's probably gouging me - check for yourself if you're in the US and you might have a much better price. (It's called the hoser discount, for a reason.)
There's half the magnet thickness, but thankfully there isn't 1/2 the flux. It's lower than it would be with 1/2" thick magnets, but we've also gone up to 28 magnets around the ring. We also haven't decreased the gap space. Back when I was learning about conversions of my own, I found that these factors compensated quite a bit for less magnet.
http://www.sparweb.ca/3_Gen_MoCo/Baldy.htmlIf this is acceptable, then the total volume of magnet is...
28 x 3 x 1 x 0.5 x 0.25 = 10.5 cubic inches.
That's pretty good for 2HP
My GE motor conversion (3HP) had a magnet volume of 12 cubic inches.
On my Baldor motor (3HP), I managed to cram on 15 cubic inches.
I was very happy with the performance of the GE, and I'm still happy with the Baldor conversion. I'm sure you'll be satisfied if you can get 10 cubic inches of magnet on your Brooks motor.