SQL Change Automation (SCA) is licensed per user.
All SQL Developers contributing to a Database project that's automated by SCA should have a Redgate Deploy license.
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 Redgate Deploy license.
Redgate End User License Agreement Schedule 2 (section 6)