By default, the SDC has a hard limit of 20 threads for the pipeline execution. Starting more than 10 pipelines will require an increase of this thread pool size via sdc.properties.
To do this, please add runner.thread.pool.size to sdc.properties with a value ~2x the number of pipelines you wish to run.
For example if you wish to run 20 pipelines, set this to at least 40.
Adam Kunicki