The Core

Great tool for figuring out sample sizes
https://www.evanmiller.org/ab-testing/sample-size.html

Most people ruin their A/B testing by "peeking" which is strictly forbidden if you want any sort of reliable results. This tells you how long you need to wait.

This is a really dense article, but it explains why peeking is so bad and how there are mathematical methods to let you peek responsibly
https://www.evanmiller.org/sequential-ab-testing.html