afaik no, you can´t set a limit for each user, what tou can do is :

- set a (small) degree for each table involved

or

- use PARALLEL_ADAPTIVE_MULTI_USER, with this the first user will NOT be able to use all the slaves, some will be reserved, and in the future, with new users trying to do parallel things, more slaves will be "removed" from the first user. But this algorithm is CLOSED, you can´t change it, you can´t specify what portion os slaves to each user...

Regards,

Chiappa