There are 173 sites in total at the time of answering, plus area 51 (a place for proposing and creating sites).
The principle of differential discrimination of ideas into separate categories for study to understand the nature of knowledge acquisition is called epistemology from the ancient Greek ἐπιστήμη (epistēmē) meaning knowledge.
Stack exchange is supposed to become a repository of knowledge, solutions and methodologies of finding solutions to problems in fields as disparate as cookery, biblical hermeneutics and computer programming (in various flavours). Each site represents millions of hours worth of research, experience and hard study by it's members - a quick google search is no substitute for this responsive resource.
Each culture's depth and variety of expression is a mark of the increase in a division and sub-division of knowledge, that is expressed here on the network, where one person cannot be an expert in all fields, as in life where a person cannot be all things to all people.
You wouldn't after all want to ask a question about how to approach a particular tricky social situation and have your question swamped by equally pressing questions about dimensional compactification or how to stop your souffle sinking.