There’s a video on the MATCH website that explains how the process works. There’s not a real percentage anyone can give you because it will depend on how they rank you, how other candidates rank them, how they rank other candidates, and individual rank lists… it’s very complicated.
If you (let’s call you “applicant red”) rank Program A #1 and they rank you #1-4 for one of their 4 spots, you would match into that program. Now, let’s say you rank Program A #1, but they rank you #5. And Program A ranks applicants pink, blue, yellow, and green #1-4 respectively and those 4 applicants also put Program A as their #1. You would not match as those spots would be filled.
However, let’s say applicants pink, blue, yellow, and green all rank other programs more highly than Program A and let’s assume those other programs still have openings. Those 4 applicants will be placed in those outside positions. Now, applicants that Program A ranked as their #5-8 will move into their #1-4 positions (assuming those candidates also ranked Program A highly and weren’t placed elsewhere a.k.a a higher ranked program on their list). In this case, you, applicant red, would match into Program A as you are now their #1 after previously being #5.