Bitbucket Pipelines has a hard cap on the number of steps it can run in parallel in any given workspace. If there are more than 600 steps running concurrently on the same workspace, which includes parallel steps, builds will also fail with the below error message.

You have exceeded the number of parallel steps allowed for your plan. Please review and reduce the number of parallel steps. 

The suggestion is to provide an option to monitor the usage of a number of concurrent steps running in the workspace.