If you're offered an interview, I'd urge you to not think about how many people they're inviting versus accepting. I'd go in ready to show that you can talk about research, your interests, your work ethic, and your past accomplishments and doing so in a fluid and professional manner. I can only speak for my program, but they only invite people that they'd be glad to have (on paper) on the sole basis that they're going to figure out who they really want to work with in the interviews/social/etc.