What is M.U.G.E.N?: It is a free, customizable 2D fighting game engine. It allows users to "port" or create characters from various franchises like Street Fighter, Dragon Ball, and Naruto, making it possible for disparate characters to fight each other.
Increasing Roster Size: To handle 800+ characters, creators often modify "slots" in the system data. Beginners can follow Instructables guides on using screenpacks that support hundreds of characters. Community Hubs for Information