I would like to create a stage musical spoof based on a movie. Where are the copyright boundaries?

Generally only the rightsholder to the original work has the right to make "derivative works" based on that original work. However, everyone has the 1st Amendment "fair use" right to create parodies of someone else's work, to comment on it and mock it. In order to parody something, you have to use enough of it so that your audience knows what you're referring to, but there's often a fine line between parodying something and infringing on its market, and you really have to be comenting on it and not just borrowing it's recognizable elements.

The fact that your intended audience is limited may keep your damages modest and make it harder for you to get caught, but it doesn't have anything to do with whether you're work is a fair use or not.

The case law doesn't have a bright line rule that says how much is too much, so you're best off seeking a consultation with an IP lawyer to go over the parameters of what you want to do.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.