You can roast as many tomatoes as you like; this is just a guide for proportions of olive oil, garlic and rosemary to tomatoes. Plum tomatoes are fantastic roasted as are smaller cherry or grape tomatoes. Don’t cut your tomato slices too thin or they’ll cook too quickly and aren’t likely to hold their juice while roasting. I do a generous 1/2-inch slices.
Preheat the oven to 200 F and lay the tomatoes out on 2 baking sheets. Sprinkle with salt and pepper and garlic and rosemary. Drizzle with olive oil so each tomato has a thin layer.
Bake for 3-4 hours (depending on the size of your tomatoes — for very large heirloom slices, it’ll take longer). The tomatoes should look a little shriveled, even browned on the edges, but should still retain a little of their juiciness.