If you rank WashU #1 and they rank you #15, it *could* be a match.
Let's say that you're applying for a program with 5 spots. From WashU's perspective, their rank list is this:
1. A
2. B
3. C
4. D
5. E
6. F
7. G
8. H
9. I
10. J
11. K
12. L
13. M
14. N
15. Medstudentquest
*IF* only 4 of the people ahead of you actually want to go to WashU-- say, persons A, F, G and N-- then you will match at WashU. If at least 5 of the others have ranked WashU highly, then you won't. Being *ranked* in and of itself will not guarantee a match there.
The computer starts with an applicant-- a randomly selected applicant, out of the 18,000+ in the Match-- and matches that person to their #1, temporarily. It then picks another random person and matches them to their #1, temporarily. Soon enough, after running through enough people, someone will get bumped. Let's keep using WashU's mythical 5-spot residency program. When the computer selects the random applicant in that huge pool who was ranked first, and who also ranked WashU first, that random applicant will be permanently matched to WashU. If the 5 slots were already filled with temporary matches, then the person lowest down on the list gets bumped.
That bumped guy is immediately thrown back into the pool, and is temporarily matched to his #2, if available.... and then, if not available or bumped, his #3... and so on.
So to answer your question there is no magic number on the rank list that guarantees you'll be matched to a particular program. Very, very competitive programs might fill in a 1:1 ratio (5 spots, first 5 on the rank list, all 5 ranked that program first, boom!-- done). Others might literally go all the way down. As you said, as long as you and the program both rank each other, and on your end none of the programs you ranked higher want you, and on the program end none of the candidates ranked higher than you prefer that program, it's a match.