Optional number of parallel flows in an activity diagram UML 2.0

Question

I have a system that starts a undecided number of similar processes (P) at different times, before terminating its activity. Some of these processes will overlap and some will end before others.
How do I indicate an undecided number of processes in an activity diagram like this:

            O
____________|_____________
     |             |
    (P)           (P)
     |             |
_____|_____________|______
            |
            O

O: start/end,
(P): process,
__: and-split/join

Here are only two processes, but I need lots of them and without writing them all out explicitly.

Answer 1

Well, this is the solution I ended up with. An expansion region in parallel mode. The text is in swedish but the relevant one is self explanatory. Expansion regions are described in detail in section 6 here.

Answer 2

Maybe the solution is to create an activity partition, which will refer to a property (having a 0..* cardinality), and containing our process but I am not 100% sure this is what you mean...