Interview with potential supervisor for graduate program. Should I keep contacting other potential professors? I applied for winter 2020 entry. The applications open in September but I was recommended to contact potential supervisors ahead of time. I emailed my CV, transcript and quick background about myself to few potential supervisors and landed an interview with one. I went to the interview and we discussed the research endeavours and my research proposal. The 45 minutes interview seemed positive and the prof mentioned me to email him when applications open. He said he will be a potential supervisor. Acceptance into the program is conditional upon securing a supervisor. Should I continuing contacting other profs or assume my prof is secured based upon the interview? I am worried if by the time applications open another potential grad might catch his attention and take my spot. Thanks in advance.
You should continue contacting and interviewing possible professors until you have an official offer of admission that you have accepted, especially at other institutions.

It seems like this professor is most likely willing to take you as a student, but only if you are accepted to that program (which likely does not depend only on that professor's opinion), and only if nothing else comes up including an applicant they like better, unexpected personal circumstances, changing financial situation in the lab, etc.

As you say, acceptance to the program is conditional on having a supervisor: that means it is necessary for admission but not sufficient. The professor has only granted you permission to identify them as a potential supervisor on your application, this is not an offer or promise of admission.