Valve covers keep the valve chamber sealed while sometimes serving as an aesthetic element for the engine. There are as many valve covers as there are rows of pistons, at least on most applications.
Valve covers tend to have a gasket seal to keep the oil inside.
Valve Cover Design
Some valve covers have the company name displayed on it in some way (etching, pressed, etc.), and in the case of hand-built engines, the main builder will have their name on it as well. They are often a way to easily identify what type of engine it is, or even the variation of a given set of engines.