I don't think you should try and update it throughout the day, it might make it difficult for people who read your comic early to get all the answers at the end of the day, and it might make people who read your comic later in the day feel like they didn't get a chance to participate.
I think whether you should do it or not partially depends not only on how many readers you have, but how engaged and interactive they are. If you have 500 readers but 20 of them always comment on your pages, then you're probably going to get at least 10 questions for your Q&A! Conversely someone could have 1000 subs, but if none of them ever comment, then they probably won't get any questions for their Q&A.
My trick for a Q&A is not to only rely on questions you actually get from readers, but also fudge and include questions that you want to share the answer to, or questions you think readers might have but aren't commenting with. If you have a fun little behind the scenes piece of info about a character, but it doesn't really come up in the story, just pretend someone asked a question about that thing and "answer" it in your Q&A.