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.
It is unwise to assume anything. Keep all options open until you formally accept an offer. Even a simple misunderstanding can leave you stranded.
But following user Bryan Krause's question, don't give the impression that you are uninterested or conflicted about the original contact. And don't give the impression that you're already committed. Stay flexible. Celebrate at the signing.
I agree with Bryan Krause. You keep contacting and interviewing until you get an official acceptance to the program. Just because the professor likes you and you two "clicked", it does not mean that they will be able to take you. It could be that some weird departmental politics may prevent them from taking students this cycle. This very issue happened to me this last cycle.
One potential adviser, who I had clicked with and had very overlapping interests with mine, informed me after the selection process began that they were lower on the level of their priority than they had expected, and as a result would not be able to take me. It was unfortunate, but it was not unexpected and reasonable. It could also be that you are definitely a favorite of them, but not enough of the department to receive an offer. The professor may also have you as a favorite, but not THE favorite. Professors recognize that the application process can be long and tedious and it is in the interest of applicants to apply to multiple programs. Even if all goes according to plan, your advisor will most likely not take offense with you having contacted and applied with other people. At the same time, you may regret not continuing to contact people if you just stopped with them.
Stay excited, but keep your options open.