I do inventory as a job and I've learned to count up to 8 at a glance. It's done by grouping: if I see six items together, rather than recognising six separate items, I see two groups of three, which I've trained my muscle memory to interpret as a six. Depending on how well organised the items are, I can count groups of up to twelve (two sets of two sets of three), but that takes a bit longer and is harder to work into a counting rhythm.