If you’re going to plant a single herb to a pot, a 6 to 8 inch pot is about the smallest to go and still get a nice sized plant. Some of the herbs would go well together and make a nice visually appealing arrangement. I’m not sure of the italian oregano’s growing habits, it might be invasive. For a mix 10 to 12 inches would do it. You could go smaller on the pots and replant more often, but I prefer more mature plants for more intense herbal properties.