400
These are the three ways in which a party could be required to be joined in an action (don't have to state answer in a question)
1. Courts can't afford complete relief among existing parties;
2. Disposing of the action will as a practical matter impair or impede the person's ability to protect the interest;
3. Disposing of the action will leave an existing party subject to a substantial risk of incurring double, multiple, or otherwise inconsistent obligations because of the interest