Shirrifs were the sole law enforcement officials in the Shire and the main branch of the Watch. Since in the Shire law was based solely on common sense and ancient tradition, it was not broken. It was the Shirriffs' job to protect the Shire from trespassers more than anything. There were a total of twelve in all of the Shire, three in each Farthing, and they were distinguished from "civilians" by a feather worn in their caps.
The only time this number was any larger was from two circumstances. The first, less serious, was the "Special Constabulary" power, where hobbits were deputized into patrolling the borders, which were called Bounders.
The second was when Saruman had infiltrated the Shire, and had taken control, with the help of ruffians like Bill Ferny. Saruman had increased the number to suppress any revolts, and arrest anybody who broke the rules. They were assigned in companies such as the First Eastfarthing Troop, though that was not enough to prevent Frodo, Sam, Merry, Pippin, and others from liberating the Shire-folk.