It's an older style of English, so the grammar rules aren't quite the same. We wouldn't say that anymore. If you can understand the meaning and enjoy the story, then that's probably all you need. It would be best not to learn to speak like someone from hundreds of years ago, unless you want to be an actor. Just to consider the actual question: I think the answer is that, if I was to say that you've liked many stupid people, then I could be referring to them collectively, or as a group. By saying many a stupid person, it emphasises that there were many times you liked a stupid person, without saying there was ever two at the same time. The wording is singular, and that emphasises the singular nature of you liking stupid people, or should I say a person at a time. That is the feeling I get from this sentence, but I could be wrong.