The number is the measurement below the bust.

an A-cup is a 1-inch difference between the measurement below the bust versus around the bust.
B-cup is 2 inches, C-cup is 3 inches, etc.
DD is the same as E, DDD is the same as EE which is the same as F. This holds valid through an H cup. After that, the interval is 2 inches, with the doubled letter being the in-between value.
This, H-cup is 8", and I-cup is 10", and a 9" difference would be an HH-cup.

The largest bra size manufactured without a special order is a size 60N.

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just to be picky, that's not quite right:

The number is the number over the bust. The cup letter determines the underbust.

if the overbust is 34" or up, add 5" to the cup size difference.
if the overbust is 32" or down, add only 3.

That means someone wearing a 36C has a 36" overbust and a 28" underbust. (36" -3" -5")

So a 32C (32-3-3=26") has the same underbust as a 34A (34-5-3=26", also)...

(and AA is 1/2" instead of 0.)