How many ways to store static variables? And which one do you prefer?

By preference, I don't.
C# is a object oriented language, and everything is class based. It has no concept of "global" variables, which is what you are describing. However, there are occasions when a single application-wide variable is a good idea - the connection string to a database for example, could be read from a config file once, and stored in a static variable. Cases like this are very, very rare though - or there is something very wrong with your design.

When that happens, I generally construct a static class, and use that to contain the static variable, along with any non-class related methods (which again are as rare as possible).