Yoma, Chapter 2, Mishnah 5. In the previous mishnayos we learned the order in which the daily Tamid offering was brought. Our mishnah comes to teach us that the number of kohanim involved in offering it was not fixed, and it varied according to the day and the season.
The mishnah opens: "HaTamid karev b'tishah" - the Tamid was brought by nine kohanim. At first glance we counted thirteen kohanim who were involved in it, but four of them were occupied with the slaughtering, the sprinkling of the blood, the clearing of the inner altar and the cleaning of the Menorah, and were not directly involved in bringing up the Tamid animal itself. Therefore, as far as those who brought up the limbs of the animal and the accompanying libations, there were indeed nine kohanim.
The mishnah adds that at times it was brought "ba'asarah" - by ten, at times "b'achad asar" - by eleven, and at times even "bishneim asar" - by twelve: never fewer than nine and never more than twelve.
How do the numbers work out in practice?
The morning Tamid on a weekday: as we learned in Mishnah 3, "hu atzmo b'tishah" - nine kohanim.
On Sukkos - ten: "b'yad echad tzelochis shel mayim" - one kohen held a flask of water. In addition to the wine that was brought and poured on the altar, on Sukkos they would also bring water to be poured on the altar, a halachah given to Moshe at Sinai. That makes ten.
The afternoon Tamid - eleven: "hu atzmo b'tishah" - the animal and its libations, "u'shnayim b'yadam shnei gizrei eitzim" - two additional kohanim who carried the two logs of wood that were added to the fire on the altar. This is a special halachah derived from the verse, and it was done at the afternoon Tamid.
On Shabbos - eleven, even in the morning: "hu atzmo b'tishah, u'shnayim b'yadam shnei bazichei levonah shel lechem hapanim" - two kohanim brought up the two bowls filled with frankincense that accompanied the lechem hapanim. The bowls were brought on Shabbos and burned on the altar, while the lechem hapanim was eaten by the kohanim.
On the Shabbos that falls during Sukkos - twelve: in addition to the two bowls of frankincense of Shabbos, one kohen also carried the flask of water for the water libation of Sukkos. This is the only time the number reached twelve: nine for the regular order of the Tamid, two kohanim holding the bowls of frankincense, and one kohen holding the flask of water.
One point worth noting: the water libation and the bowls of frankincense were brought only in the morning, and not at the afternoon Tamid. For at the afternoon Tamid, even on Shabbos, the two logs of wood were brought, and if the flask of water and the bowls of frankincense were added to them the number would rise to fourteen. From here it is clear that the flask of water and the bowls of frankincense were not brought in the afternoon, but only in the morning.