The ABC medical drama was recently picked up for 13 episodes, thought to be the show's final run.
However, Rhimes told TV Guide that she is not currently planning to wrap up the series, which stars Kate Walsh as Dr Addison Montgomery.
"I have been told that I'm not allowed to think of us only doing 13 episodes," said the showrunner.
Rhimes also defended her recent decision to drop series regular Tim Daly, who has played Peter Wilder on the show since it began in 2007.
"There's a lot of budgetary issues on that show," she said. "We need to make sure the show is viable and that we can produce it. I feel as though Tim's character had a wonderful journey."
It was recently reported that Kate Walsh may also depart Private Practice next season, having only signed a contract for 13 more episodes.
> Private Practice Shonda Rhimes: 'I refuse to accept it's the end'
Watch a promo for the Private Practice fifth season finale below: