Where can I configure the maximum number of threads that are used when a seed tiles request is used? The default seems to be 16
The web interface of GWC seems to use name "task" where it probably should be "thread". Task means a job that aims at seeding the area of the given bounding box and it can use one or more threads.
Anyway, the documentation in http://geowebcache.org/docs/current/webinterface/seed.html and http://geowebcache.org/docs/current/webinterface/seed.html is correctly speaking about threads.
The default number of threads for a task is 1 and it can be set with the drop down menu item which has misleading title "number of tasks to use" or through the REST API. The same misleading name "task" appears on the status page which looks like this when 1 task is running with 4 threads.
Your screen capture informs that you have started 19 separate tasks, each of them running in one thread. All seeding requests have just been started because time counters are in Estimating status. I suppose that you have started the tasks with REST.
As far as I know it is not possible to limit the number of concurrent tasks and threads in GeoWebCache but it accepts all new requests even if there are already lots of old tasks running. That can make the whole service to stuck.
As per user30184's answer, the script doing the (re)seed calls has to send them in a delayed mode to make sure all the tasks are not run at once.