Zero-One-Infinity Rule

Zero-One-Infinity Rule: When designing software systems, the only reasonable numerical limits are zero, one, and infinity.

Article: Zero One Infinity Rule

Zero-One-Infinity (ZOI) is a software design rule of thumb. It states that a programmer should not place random limits on how many instances a given entity should have. The only reasonable limits are zero, one, and infinity. In other words, you will either need zero of a thing, one of a thing, or an arbitrary number of the thing. Any other arbitrary limit increases the cognitive workload for both the users and the software designers.

A simple example is the file system's folders (directories). The topmost folder must have "zero" parent folder. Each subfolder must have exactly "one" parent folder. Each folder can have "any number" of subfolders.

Some people also use this as a heuristic outside of software design. For example, if you're going to believe in more than one universe, you might as well believe in an infinite number of them. Same for estimating how many alien planets there are.