If you don't need advanced features like multisite handling or cloud functions, the standard version of MAMP is completely free to use [1].
If you already own a license and need to verify or recover it, use these official methods: mamp pro 7 license key verified
MAMP PRO 7 requires a paid license, which is a from version 6. Verification and registration can be handled directly within the application. If you don't need advanced features like multisite