The short answer
Five is plenty in the first couple of years. New ARR added this month, gross and net revenue retention, CAC payback, pipeline coverage for the coming quarter, and win rate on qualified opportunities. Everything else, and there are dozens of tempting candidates, is either derived from those or too noisy to act on at your volume. Track them monthly in one spreadsheet, keep the definitions written down next to the numbers, and never change a definition without noting the date, because a redefined metric quietly destroys a year of trend. If you can only look at two, look at net revenue retention and CAC payback: they are the pair that most reliably predicts whether the business compounds.