Knives are tools. Tools that are excellent at a wide variety of tasks are rare. Yes, you can make do with a single chef's knife, but I don't see why would you want to. Even for basic kitchen activities, ignoring specialized tasks, I would want to have on hand at least three knives -- one big (e.g. a chef's knife), one medium (usually called a utility knife), and one small (a peeling knife).

Oh, and a steel is not used to sharpen knives, it's used to straighten out the knife's edge. To actually sharpen you will need a sharpening tool -- there is a large variety of those, differing in capability and the required skill.