This is from the story "Evans Tries An O-Level" by Colin Dexter", page 2, last paragraph. How is it a joke?
"They seem to have a lot of parsons there, don’t they?” The two men chuckled good-naturedly.
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.
St Mary Mags is St Mary Magdalen Parish church in Oxford. There has been a church on the site for over a thousand years and hence it is a significant part of the past and present of Oxford.
They currently seem to have over 20 masses or prayer services each week, as well as having outreach to the community and colleges. I guess they probably need quite a few clerics to keep all that going, and probably had even more when the story was written.
I read the purpose of the exchange being to show the two men bonding a little by indicating shared knowledge of the local area, but more importantly for the reader, it sets up a reason for no-one being familiar with a particular parson from there, because there are so many. If the invigilator had been a minister from a single-preacher church, they would be more likely to know him on sight. This tilts the odds in favour of the success of the escape plan.