If you have a project in mind, read the pattern and the pattern should indicate how much yarn is needed to complete it. If you find yarn that you love but do not have a specific project in mind, buying two skeins would most likely allow you to make something like a hat and gloves set, or you could purchase more skeins to give yourself more options.
Yarned and Dangerous also sells patterns, which would allow you to choose a project to use that special yarn that speaks to you while shopping in the store.