Ebenezer saying he has had 3 dozen apprentices is explained by his time as the Chief of the Wardens. A wizard goes through regular apprenticeship, decides to become a Warden, then get shipped off to Eb for a year or two of OJT. We know that Wardens get trained by the Chief of the Wardens per White Knight. Eb probably spent decades, or even a century, as a veteran Warden so he had plenty of time to do it even if he did it one at a time. It even explains why Harry was allowed to become a Warden without any official training. He spent years with Eb, he's a licensed PI, and has clear success performing the job of a Warden so he's got a GED in Warden training.