The world is an awfully big place. It really is. Seven continents, thousands of languages, and an endless variety of cultures. There’s really only one dominate species on the planet (us humans), but we have thousands of years of written history. The goal of world building in a fantasy literature environment is to provide a background to the story that is as believable as the real-life background that our very real world provides to the stories and events that take place here. However, trying to be as complete and exhausting in detail as the real world is impossible and ill-advised. One can only work within certain limits, within which you, as the author, have free reign. You can develop a world with but one continent or just a vast collection of islands, with one language and culture, or many, and so on. What I’ve found in my own writing, though, is that there is an upper limit to the number of facets that can be adequately described. As a rule, the writer does a certain amount of world building, but only uses about 10% of that material for his/her book. That may seem like a waste, but it really isn’t. Although you only use 10% of your world building materially directly in your book, the other 90% does influence the work: It provides the context for that slice of material that you do use. You will draw on this other 90% in subtle, yet important, ways. A conflict between two religions can be spelt out in detail in your world resources and because you know the why’s and wherefore’s of this conflict, you can incorporate touches of it in a striking, realistic manner that does not overwhelm the reader with unnecessary details, nor pull her out of the story as she is reading.
The important thing to remember when using the material in the book is, as my title suggests, limits. The human mind can only handle so many details at once (there’s a reason phone numbers are only seven digits long—barring area codes). I think a good number range is 3-5 for any particular aspect of world building. You can, if you wish, develop thirteen different cultures on your world (in fact, to be realistic, there should probably be many, many more), but I think you would be ill-advised to use more than five in a particular novel, or say, seven, if it is a very long series like “The Wheel of Time” by Robert Jordan/Brandon Sanderson. For a single stand-alone book, in fact, I would recommend limiting yourself to three. That lets you develop each one fully without overwhelming the reader. Remember, it is far easier for an author who has written, re-written, edited, and re-edited the same work several times to keep all the details straight than it is for a single reader who only reads the book once. The goal is to strike the appropriate balance, producing a world rich in detail, but not overwhelmingly complex and confusing. The same can be said for religions, races, creatures, or what-have-you. A smaller number of well-developed world building aspects will probably serve you far better than a hodgepodge of everything under the sun.