You are using the 4" disks right?
If so, I'd start out with about 5 of the 4" disks.
If they are 3" I'd say about 7 on each side.
Remember to face the fins towards the back, and the dish towards the front...you'll see what I mean when you install them. Don't over tighten the screws or you will crush the disks together and block flow. You want them tight enough not to rattle...but that's it. The screws should be drilled for wire, and use some anti-seize on them. They can be a real bitch to get out if you over tighten them and the SS screws gall.
Don't be tempted to put a big stack of disks on there...they don't need it. A meg does it's job with it's shape...not the big hole in the back.