Really depends on size and weight of trees. The bigger and heavier the trees, the longer the wedges you need. They're almost all 1" lift, so the longer they are, the more mechanical advantage you have when driving them in. So, if you're wedging 3' wide 150' tall redwoods, well, you're probably going to need those 10 or 12 inch wedges. That's a lot of weight to lift. People in those area's probably don't even mess with 5" wedges.
Round these parts (midwest) I rarely use a wedge over 8" and most of the time I'm using the 5" ones. I have a couple of each, even 10" and 12". Those stay in the pack generally unless I find myself into a big conifer. I take 3 of the 5" and 1 8" with me when I'm falling. I've never had a problem driving 5" wedges, I have had the problem where I needed to wedge a smaller tree, took the smallest face cut I could, still bottomed out an 8" wedge and that's all I had on me...not a great situation. We just don't have super big trees in this area.
Get a few of all of them, use them, then you'll know which ones to have more on hand and which ones you can relegate to the 'sometimes' bucket.