Currently, if a 'bot' account can access the Pipeline, it needs to claim a seat similar to a traditional user on the account.

Please allow the option to create a bot/pipeline-specific account that does not count towards the Bitbucket user total.

https://community.atlassian.com/t5/Bitbucket-Pipelines-questions/Allow-CI-Pipeline-to-be-added-to-branch-permissions/qaq-p/1380522