Q. How many neutral wires can I install on a single terminal in a panelboard?
A. According to 408.41, each neutral conductor within a panelboard must terminate to an individual terminal. What's the thinking behind this rule? If two neutral conductors are connected to the same terminal, and someone removes one of them, the other neutral conductor might unintentionally be removed as well. If that happens to the neutral conductor of a multiwire circuit, it can result in excessive line-to-neutral voltage for one of the circuits, as well as undervoltage for the other circuit. See 300.13(B) for details.
This requirement doesn't apply to equipment grounding conductors because the voltage of a circuit is not affected if an equipment grounding conductor is accidentally removed.