Parsons are funny. They wear silly robes and have very little power. They have a low rank in the Church of England, responsible for local administration. Even the word is a bit silly (not least its association with the parson's nose, which is a chicken's bum.) The Church of England is a common subject for English humour (e.g. _The Vicar of Dibley_ ), because of the declining importance of religion in British society.
But beyond that, the remark in the story doesn't seem to have much meaning. It doesn't seem like it can be explained from the material in the story, and certainly the reader doesn't know anything about the number of parsons at St Mary Mags (presumably an affectionate shortening of St Mary Magdalen's). Most likely it is an in-joke to show the warm relationship between the characters.