This is a request for a rule which is already provided for Flex, PHP, Objective-C, C++, and Java.
The justification for this rule is that method count (especially for those in a classes public API) is a good measure of complexity in that class.
Existing implementations of this rule come with a count as well as a flag for whether or not to include non-public methods in the evaluation. I would argue that an ideal implementation would have these as two separate rules, as well as possibly another to differentiate between concrete classes and interfaces, but for the sake of simplicity and conformity I think the existing definitions will suffice for most.
Compliance examples left absent as they are trivial.