yes, is possible. 'Dynamic assignment' can be done with assignmenthandlers, and 'dynamic number of tasks' can be done with a custom fork handler (see the wiki). You can combine them if you like.
So things like 2 out of 3 different people have to approve a certain thing can be implemented
Thanks for the reply.
Can you please point me to the wiki (or any other documentation) where custom fork handler is explained? I would now like to create a process definition to try these things out before deciding on jBPM.