maximum 8 instances are seen active in the web UI, if some of the instances are completed or terminated, remaining instances will be appeared
Can you look at the Start and End times of these executions once they all complete and share maybe?
There is no restriction to X number of workflow executions that can run concurrently so I am a bit confused given the current info you provided as to what is causing this still.
Can you also give bit more info on your workers? Which Temporal SDK do you use and what WorkerOptions do you set?