SQL Change Automation (SCA) is licensed per user.

All SQL Developers contributing to a Database project that's automated by SCA should have a Flyway Enterprise license. SCA is not sold as a standalone tool.  

You can then install the PowerShell cmdlets on as many servers as required by your CI/Release process.

Using the SQL Compare and SQL Data Compare command lines on a server also requires a Flyway Enterprise license.

Related documentation:

Changes to distribution of command line

Integrating the command line with applications

Redgate End User License Agreement Schedule 2 (section 6)