It is unknown whether a jersey number was given to a certain person or position, but Harry Potter often wore the number 7 as a Seeker.
It also depends on the year and the Quidditch season. The Quidditch jerseys have changed their general appearance several times. The colors of the teams have also changed shades.
From 1991 – 1993, the robes that were worn by the players had shorter sleeves and tie fastenings on the front, while the trousers they wore were white. From 1993-1994, the robes had numbers on them and featured the players last name on the backs.
In 1996-1997, during tryouts prior to the season, the jerseys featured a number on both front and back but did not have their names. These jerseys had a back side that draped all the way down to the players ankle and the front stopping around the waist giving the appearance of a cape.