Would i be considered claustrophobic because if there are a lot of people in the elevator, i take the stairs?

Probably not: Claustrophobia is thought of as a situational type of specific phobia. A phobia is a significant and persistent fear that is excessive or unreasonable and set off by a specific object or situation. Claustrophobia is a fear of closed tight spaces. I think if you had claustrophobia you'd probably have an unreasonable fear of elevators even when they're empty. A preference is different from a fear.