The number underneath that letter is the first page of entries for people with that set of initials. Decedents within that group are then listed chronologically. You should always check Orphans Court after using probate records. Chester County Archives has abstracts to wills and administrations for 1714-1844. The abstracts are summaries of the wills and administration bonds.
Every name is indexed.
The abstracts do not include names listed in accounts of administration. The abstracts are the best way to narrow your search before requesting numerous files of a common surname. After you locate an abstract that interests you, request the original file.