The end result may seem simple, but knowing how to express what you need in regular expressions is not so simple. Don't worry about it. Even people who write them all the time tend to look up a reference for all but the most basic expressions.

There are many ways to do what you're after, but here is one that should work in Workflow.

\((\d+)\)

\(    = Start by matching the literal character "("
(     = Everything after this will be captured into a group
\d+   = One or more "digits"
)     = the end of the group
\)    = Match the closing ")"

Basically, only the digits inside of the inner parentheses will be captured by Workflow. You'll want to test it out on a variety of document names, but this should work for you.

If the number inside the parentheses is always the same number of digits, you can do something like this.

\((\d{6})\)

\d{6}    = Match exactly six digits

\d{6,}   = Match six or more digits

\d{6,8} = Match from six to eight digits