The Cheun Jen Sect's Big Dipper 7 Stars Formation was originally designed for use by seven people...namely, the seven disciples of Cheun Jen Sect Founder Wong Chung Yeung.
During the course of LOCH, the Cheun Jen 7 Disciples lost Tam Chui Deun. They tried to plug the gap with third generation member Wan Tze Ping, but it just wasn't the same.
Halfway through ROCH, Ma Yuk died from old age. The Cheun Jen Sect Second Generation disciples were down to five by the time that the Golden Wheel Monk and his Mongol allies attacked the Chung Yeung Temple.
Nevertheless, the remaining five had a powerful formation that they were able to use to hold off the Golden Wheel Monk (at least until Fok Do intervened). Did they redesign their fighting formation to accommodate five rather than seven fighters? All of the Cheun Jen Sect's battle formations were arranged in multiples of seven. Re-arranging it to be effective for five could not have been an easy task.