This is a really difficult question to answer.

Some of my answers are of the sort where practically each sentence contains a fact that's drawn from a different book or article. Others, I'm drawing almost exclusively on one source. So from that perspective, you could technically get lucky and give a sufficient answer having read one book.

The wrinkle, though, is that you have to have read enough to know that the answer you're giving is "in depth and comprehensive." That is, you have to know how that book's evidence and conclusions play into the larger understanding of the time period and topic in question. How much reading does that knowledge take? I really don't know. Especially because follow-up questions can sometimes range very far afield of the OP.

The other complicating factor is that academic history books don't tend to pursue arguments that align with the way the AskHistorians community tends to pose questions. (Often, the nature of the questions is the first tip-off that something is a homework/essay prompt, hehe). To simplify things, I'd say AH gets a lot more "what it was like to live in the past"-type approaches. Writing a quality answer to these questions takes not just reading but a certain ability to mine an argument-based book or books for facts to realign into an answer to the question.

I'd say that for the most part, you'd want the equivalent of an undergrad-level focused course (i.e. not "World Civilizations II") on a topic's worth of reading, with the very important caveat that some of the best AH contributors have no formal academic-history experience. :)

Thanks for tagging me, /u/caffarelli!

So for the Spanish Civil War I think you'd need to read an absolute minimum of three books--two general histories and one specific study. To give good answers you'd have to read more--maybe half a dozen books and a few journal articles. To give really good answers you'd have to read as much as you could get your hands on. More importantly than all of the above is retaining all of the information that you came across so that you could not only communicate it to others but also put it into context in the larger historical debates on the topic. Also doing primary document research would be a big help (so speaking Spanish would be very important, or a dozen other languages if you were looking at a different aspect of the Spanish Civil War). Caffarelli is absolutely right that to be able to argue for your own answer versus others would likely require reading a few dozen books/articles/primary documents.

Since Caffarelli asked me directly, I've read a dozen books on the Spanish Civil War, likely double that. I've read mentions of the war in another couple dozen books, but those only cover the war in relation to another topic (Pius XI or Pius XII, for instance). I've done primary document research and translation (though my Spanish is too limited to do a ton of that), and my undergraduate thesis was on the subject. That's not counting a fairly large number of journal articles as well. And if i'm honest I would say that I have a basic understanding of the Spanish Civil War, a fairly good understanding of the subject of anticlerical violence in the conflict, and a good level of knowledge about Vatican involvement. For the Republican side of the conflict i'd be relying on /u/tobbinator or others, and for the Abraham Lincoln Brigades i'd probably be yelling for /u/Georgy_K_Zhukov to lend his knowledge.

As to the OP's question about

What kinds of books, how old can they be before they are outdated?

The Spanish Civil War is interesting in this regard. Paul Preston (an expert on the subject) claims that there has been nearly as much English-language scholarship on the Spanish Civil War as there has been on WWII. That is staggering to me. And the scholarship has come in a number of waves. There was contemporary study of the conflict, and much of it is highly biased. As history these works can leave much to be desired, but as a source of what people felt at the time they are invaluable. Over time more balanced narratives emerged, but Franco's regime kept a number of sources inaccessible. After Franco's death the modern histories of the war as well as better access to documents and archeological work.

So there have been a number of different periods of writing about the Spanish Civil War, each of them with their own advantages and disadvantages. As to the age of particular works, one might be tempted to discount Hugh Thomas' The Spanish Civil War because it was published back in 1961. However, it has been updated over the years and continues to be one of the best one-volume histories of the war. If you haven't read Thomas, you're likely not an expert on the Spanish Civil War (in my opinion). You'd also need to stay up to date as much as possible, so any new works would have to be in your wheelhouse as well.

I hope that addresses your questions, but the Spanish Civil War is among the more complicated subjects one could dive into. There are so many facets to that conflict that one could probably spend a lifetime learning about it and still not have mastered it all.

Everyone else's answers are fantastic, but I would also like to add one thing - that simply reading history books or journal articles is not nearly sufficient in many (most? all?) subjects. Particularly in the disciplines which deal with earlier time periods, having an understanding of the historiography, as /u/Georgy_K_Zhukov mentioned, and also a close familiarity with the primary sources is key. See, what most people think of as a "history book" is a synthesis of many different arguments, derived from articles, other people's books, and even works from the past that new information, discoveries, or understanding has made outdated. Part of moving from undergraduate-level generalist status to expert status comes not just from having a passing familiarity with the arguments but also in being able to make your own arguments using the source texts (or archaeological finds, as the case may be) and being to situate your own opinions within the historiographical record.

Everyone has provided some great insight, and I'm not sure how much more I can add, but one thing I want to focus on is "How many books would I have to read on this topic to answer questions regarding it". Simply put, there isn't an answer. Some questions you can answer quite succinctly having read a single book on it, while others might require a synthesizing a small library to do adequate justice. So while you should be reading voraciously, becoming an expert on a topic isn't dictated by your "read" shelf on Goodreads, but rather on how you apply what you have read. Someone who has only read a few books on a topic, but has a great grasp of their content and can engage with them and use them critically, is likely going to be able to write a better response than someone who has read many more, but can do little more than list facts and figures that they recall from their reading.

So that is to say, read all that you can, but make sure you are working on developing your skills as an historian while doing so, not just memorizing facts. Think about what you are reading, compare it to other sources that you have read. Take notes - not just highlighting things, but leaving your own thoughts on what you just read - did you find the author's argument compelling, if so why, if not, where was it deficient? Make sure you're not just reading a single narrative, but try to familiarize yourself with the historiographical debates in the field - those big tomes are great, but make sure you're reading journal articles and the like too. You want to be able to utilize sources critically and engage with them.

So yeah, that's what I would recommend for you to think about. There is no number that you reach and you level up to "expert" rank. It is a fluid process that is less about gaining rote knowledge and more about gaining the skills to apply it well.

You know, for funzies, I made a quick count of the books on my Goodreads account tagged "eunuchs," "academic," and "read," and it's about 50. But then I've read a lot more academic articles, in the hundreds. And probably about 20 books purely on opera and not castrati, for background. So get reading!

But, to echo /u/sunagainstgold there's a few different types of questions here and they require a variety of background to answer, some are very specific and focus on essentially random historical details like this, and really you just need to know That One Thing to answer them. Obviously you don't need to read 30 books to know one specific factoid. Others are Sun's favorite "what was life like in the olden days," like so, these take a fair amount of background knowledge to answer, but not a staggering amount of analysis usually. People either wore pajamas, did not wear them, or wore them sometimes and not other times depending on reasons, so it's not a backbreaking amount of work to argue for one of those answers. Others make you come at history a bit sideways to anything you normally see in academia, these questions are usually the why/why not/how questions, like this (which as you can see was kinda too complicated to answer), or this. These take a lot of background knowledge to answer, as there's going to be multiple possible "correct" arguments, and you (Intrepid AskHistorians Historian) must know a fair amount about each possible argument, enough to not only select your own answer, but to coherently argue against other answers and for your own. So that's when you need to have read 30-50 books.

For Spanish civil war, I know off the top of my head /u/domini_canes has read a lot about that, how many books have you read Domini?

Fascinating, you all do great work, many thanks!