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 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 SQL Toolbelt or Flyway license.
Changes to distribution of command line
Integrating the command line with applications
Redgate End User License Agreement Schedule 2 (section 6)