What is the optimal number of worker tasks to specify in MySQL Workbench for a speedy migration?
There is a one worker per table limitation, so the maximum number depends on the number of tables you are migrating.

Notes:


A decent maximum is the number of cores on your system
If you specify a large number but have fewer tables, then only the needed threads will be launched
If the network saturates, like maybe 20 tables with 20 workers, then the copy may actually be slower


So, there isn't a simple answer but hopefully this helps. For additional details, see the related wbcopytables documentation.