YJSwan

In addition to what GeorgiaTechPhd said, you can be very honest with the professors about your options. They know how the admission process works and understand that everyone wants to wait for the best option. You can even share with them which schools you're waiting for. One of my professors in grad school emailed with an admitted applicants multiple times. The professor helped him with scholarship and everything. Then the applicant was a little embarrassed to tell the professor he just got admitted into another school. My professor said to him, "Why are you even hesitating? That's a good school. Reject this one and accept the other before time runs out." He was truly happy for that applicant and we lost a great candidate to a better school. The moral of the story is that good professors always have the best interest of students at heart. They don't get offended just because you want the best result. And, if someone does get offended just because you want the best result for your life, it may as well serve as a warning that they may not be good advisors anyway... Courtesy and honesty. These two can never go wrong.

Some do, based on my experience. A few years ago I got into a master's program. On the first day of every course, every professor told us about how our director and all professors were freaking out on April 15 because there were too many of us coming to class. Our department has the admission data of the past years, and the admission to enrollment ratio fluctuates within a certain range. Each year, the department gives a prediction of this year's admission/enrollment ratio. Then, based on that prediction, the admission committee determines the number of admission, targeting at an expected number of enrollment. Our program doesn't seem to use a wait list system. Our director once mentioned how he thought a wait list was a torture to applicants. For reasons still unclear to this day, our year went rogue. More people decided to enroll than predicted. Huge salutes to our professors that decided not to undermine the quality of the program. They decided to take up much more work and teach each course twice a week, so that we had two sections with a single digit class size. (I was in Arts, not Humanities, but close? Closer than STEM? ?) Of course, that was only my school. It is very possible that some schools use different strategies.