The district council when setting up the Parisb will have held a governance review which established the need for wards and the population or isolation from the main part of the parish that will decide how to distribute councillors.
A parish council, can request the district council conduct another review if they feel the numbers seem to be unbalanced. But the district does not have to do so. Residents can also petition the district council to conduct a review, your district council will be able to tell you how many electors need to sign for it to be valid in your area.