Shevi'is, Chapter 3, Mishnah 2. In the previous mishnah we learned that at a certain stage of the Shemittah year one may gather piles of manure in the field, so that the field can be fertilized after Shemittah. Now the mishnah comes to establish the measure: how many piles may be set up, and how large each pile may be.
"Ad kamah mezablin?":
The mishnah asks "ad kamah mezablin?" - up to what measure may one gather the manure, and how many piles may be set up? Its answer: "ad shalosh shalosh ashpatos l'veis se'ah, shel eser eser mashpelos, shel letech letech". There are three measures:
"shalosh ashpatos l'veis se'ah" - in every area of a beis se'ah, which is fifty amos by fifty amos, one may set up three large piles of manure.
"eser eser mashpelos" - each pile contains ten containers of manure.
"letech letech" - each mashpelah, that is, each container, holds a letech, which is fifteen se'ah - half a kor.
It emerges that each pile contains one hundred and fifty se'ah of manure (ten mashpelos of fifteen se'ah each), and in every beis se'ah - fifty amos by fifty amos - one may set up three such piles.
"Mosifin al hamashpelos v'ein mosifin al ha'ashpatos":
The mishnah continues and rules: "mosifin al hamashpelos" - one may increase the number of containers in each pile, and one may even set up a pile larger than the stated measure. But "v'ein mosifin al ha'ashpatos" - the number of piles themselves may not be increased, and is limited to three.
Since one may add to the mashpelos, why was the measure of ten mashpelos per pile established at all? The reason is that one may not set up a pile of fewer than ten mashpelos: when the quantity is small, it looks as though he is fertilizing the field. A certain quantity is needed in each pile so that it will look like a pile - a large heap and a collection point for manure, and not like fertilizing the field, which is forbidden during the Shemittah year.
It is worth noting that even though the Tanna Kamma ruled that one may not add to the three piles, there is an opinion that one may add even to the piles. According to this opinion, the three piles per beis se'ah are a minimum measure and not a maximum measure, and one may set up even more than three.
In summary: in this mishnah we learned the measures for gathering manure during Shevi'is - three piles for each beis se'ah, each pile of ten mashpelos, and each mashpelah in the measure of a letech, which is fifteen se'ah. One may add to the mashpelos but not to the number of piles, and the measure of ten mashpelos was established as a minimum, so that the pile will look like a collection point for manure and not like fertilizing the field, which is forbidden during Shemittah. We also noted the additional opinion, that even the number of piles is not limited to three.