Hey there.
SonarCloud counts the largest branch of all projects towards the license limit. It seems like your main branch pushes you over that limit while other branches don’t (and most likely, short-lived branches and PRs will never be the largest branch in your project).
You can purchase more Lines of Code, or try adjusting the analysis scope to exclude files (sonar.exclusions).
Adding /d:sonar.verbose=true to your analysis should give you a really good idea what all files are being indexed, and where you might be able to exclude some (third-party modules, etc.)