Many years ago I was involved in the establishment of a new parish council locally and believe there are government guidelines (not sure if it's in any legislation) which recommends the number of councillors relative to the population of the area. This suggests that any change in the number of councillors should be related to changes in the population of the parish area. For example if a large development has occurred it might be reasonable to expect a larger number of councillors. Similarly, if a boundary review has reduced the population significantly it might be expected that councillor numbers might be reduced too. However, I don't think the number of councillors can be changed at the whim of the principal authority; it would have to be part of a wider electoral review which itself would include formal consultation.