The information you received about the "purge day" was correct, and if at least 19 people currently attending the class did not, for some reason, pay their tuition, they'll be removed from the class list. There is also a "purge" of students who sign up for a class, appear on the class list, but do not show up. This, however, can happen about twelve days into the term, and can be too late for you to join the class. I agree with the advice that you could try asking the professor to "over-tally" and allow you to enter the class, but you should NOT use e-mail to do this. You need to make the request IN PERSON if it has any chance of succeeding. It is far too easy for a professor to delete an e-mail and forget about it. It's entirely a different matter if the student is standing right in front of him, asking for a favor. That real human being is much more difficult to ignore or turn down. Although there are no guarantees in life, it is a wise policy to avoid e-mail for any academic matter you consider important.