Few enough that a rep can update a record in about ninety seconds, which in practice means roughly five on an opportunity: account and primary contact, next step with a due date, stage with one written entry rule, expected value and expected close quarter, and two lines on what was discussed. Test every other candidate with one question: what decision changes based on this field, and who makes it? If nobody can answer in ten seconds, delete it, and remove it from the layout rather than making it optional.