Some prefer to stock the tank full of rocks, some like me like just a few show pieces in the main tank, leaving most of the uninteresing rocks to the sump.
If you are buying from the LFS, then you don't need to buy them according to any frigging rules. Just a few pieces, and see how you like the way it looks in the tank. Then get a few more pieces later, and a few more later. Stocking the tank should be an art, not a chore......
But, if you are buying rocks from MO, Well, I'd suggest you order just a few to start off, like 3/4 Lb per gallon. Stocking LR one week or two apart won't significantly impact the cycling schedule. LR are expensive. You don't want to get too much and end up with a piece or 2 rocks that are too expensive to throw away.