Five numbers cover a small business: leads by source, cost per lead by paid channel, conversion rate of your main page or form, review count/rating trend, and revenue attributed to marketing (even roughly). Everything else is drill-down.
The discipline that makes five numbers powerful is sameness: same definitions, same ruler, every month, written down. Trends surface problems long before any single month's number does.
Skip the vanity aisle entirely — impressions, followers, 'engagement' — unless one of the five numbers degrades and you're diagnosing why. Dashboards exist to prompt decisions, not to feel busy.