There isn't any specific number of payments that have to be missed before taking legal action.The fact that he is not making the payments required by your divorce decree would be grounds for pursuing contempt of court. You will need to file an Order to Show Cause to schedule a hearing for contempt of court.  Prior to filing that document, call the court clerk to schedule a hearing date and include the date/time/department in the Order to Show Cause. With the Order to Show Cause, also file your declaration signed under penalty of perjury stating the facts in support of your contempt of court claim and any other documents or other items as supporting evidence of your contempt of court claim along with a proof of service (court form). File your documents with the court and mail a copy of your filed documents with the proof of  service to your ex to provide him with notice of the hearing.  The proof of service verifies the date of mailing.Prior to filing your documents with the court, ask the court clerk if there are any other required documents to file for contempt of court because the required documents may vary from state to state.