How many tests should you run? The method, not the magic number
The rule: choose by risk type
Deterministic (link, route, file, calculation) → 1 test. Unstable (network, API, latency) → 3 to 5. Critical (security, money, whole system) → 5 to 20. Statistical / load → 30+, rare.
The infinite retest
The best "number" is not a number. It is an automatic test book, relaunched every hour. In one sentence: 1 if deterministic, 3-5 if unstable, 5-20 if critical, 30+ if statistical — with an automatic test book on top. No more invented numbers.