Which has priority - role or user?

The answer is 20, not because one takes priority over the other, but because one doesn't take away the permissions of another.

It's a generous permissions grant, much like you can be a member of two different roles and it only takes one of the two roles you're a member of to have a permission granted for the user to have access to it.

If you want the user to have a max of 10 sessions, then their role needs to have that limit, then individual users would have an exception of 20 or you can create another role that has 20 as the limit and assign two roles to people you want to have 20 instead of 10.