Someone who owns a herd of at least ten animals must arrange to have them tithed. If, however, the animals are not in a single herd, even if the farmer owns many animals there will be no obligation to tithe them in the event that no single group contains ten animals or more.
What is considered to be a herd?
Animals are joined for purposes of tithing if the distance between them is no greater than the distance that a grazing animal can walk and still be tended by one shepherd. What is the distance over which they can wander while grazing? Sixteen mil. If there was between two groups of animals a distance of thirty-two mil, they do not combine for the purpose of tithing. If, however there was a herd in the middle of the distance of thirty-two mil, he brings all 3 flocks into one shed and tithes them at some point in the middle. Rabbi Meir says: the Jordan River is regarded as forming a division with regard to the tithing of animals.
Rabba bar Sheila explains in the Gemara that based on a passage in Sefer Yirmiyahu (33:12-13), we know that a herd depends on the ability of the shepherd to see all of the animals, and that the Sages had a tradition that it is possible for him to keep track of them up to 16 mil.
Although the word for the Mishnaic mil is based on the Roman mile, it is shorter than that distance which is equivalent to 1,000 steps (about 1,490 meters). A mil is a distance of 2,000 amma (cubits), which was used in the laws of eiruvin, and today is estimated between 960 and 1,200 meters. Although some suggest that the shepherd can literally see animals over a distance of 16 mil when he stands in the middle and only has to observe eight mil in each direction, according to Rashi it appears that the intention of the Gemara is to say that a qualified shepherd can control his flock – even if he cannot see all of the animals – over a distance of 16 mil.