I wonder why crucial doesn't offer a 32gb package on their site. They only offer 16. You can buy 2 sets of the 16gb, but aren't they specially matched or something when you get them all from the same package?
You have 4 slots in the imac. As long as each ram piece inside has the same brand and size it's fine. So 4x4 gb, 2x8gb or 4x8gb. Probably crucial doesn't sell 32gb packs because its pointless since you can buy two packs of 16gb and 32 is yet not a mainstream amount.