each professor usually just takes 1 student so just divide 1 by however many people are interviewing.
The odds seem like they'd be in your favor but remember that these people who get interviews are super qualified also and you don't know what kind of relationship they have with the faculty member already. For instance, could already be working in their lab, currently work with one of that faculty members old students, just a lot of factors can come into play which make it still so competitive. Don't even think about your odds just do the best you can and you can't be upset with yourself