This is the limit on the number of concurrent workflow instances that can be initiated from the UI in a single request.
The Warning dialog comes from here :
/libs/dam/gui/coral/components/admin/timeline/events/workflow/clientlibs/workflow/workflow.js
You can select a folder which contains all the Assets and this will initiate a single workflow instance to process them collectively.
By default, this limit is set to 15. To modify it, you can overlay below, but be sure to consider the system's performance impact.
/libs/dam/gui/content/assets/jcr:content/rails/timeline/granite:data
