The term "Sandak" refers to the person given the honor of holding the baby during a Brit Milah. Serving as a Sandak is considered a great Mitzva, deemed equivalent to offering the Ketoret (incense) in the Beit Ha'mikdash. (Hida- Petah Enayim, Yoma 26A)
On the basis of this equation drawn between serving as Sandak and offering incense, the Rama (Rabbi Moshe Isserles, Poland, 1525-1572) writes in Yore Dea siman 265, based on a ruling of Rabbenu Peretz (one of the Tosafists, France, 13th century), that one should not serve as a Sandak for more than one child in the same family. The Gemara states that a Kohen who offered incense in the Temple was rewarded with wealth, and therefore, in order to allow as many Kohanim as possible to earn this reward, no Kohen would offer incense more than once in his lifetime. By extension, the Rama rules, a person should not serve as Sandak for two children of the same family, and should instead allow others the opportunity to earn the reward granted to those who perform this Mitzva.
The Gaon of Vilna (1720-1797), however, disagreed, and allowed serving as Sandak on multiple occasions even for children of one family. He argued that according to the rationale underlying the Rama's ruling, we should allow people to serve as Sandak only once in a lifetime, even for children of different families, and nowhere do we find such a practice mentioned or observed.
Rabbi Yehuda Ha'chasid (Germany, 1150-1217), in his "Sefer Chasidim," likewise writes that one should not serve as Sandak for more than one child within the same family. However, there is some question as to whether the laws presented in Sefer Ha'chasidim are universally binding. Some claim that Rabbi Yehuda Ha'chasid established these laws only for his own family, while others assert that they are binding only upon Ashkenazim. (Yichaave Daat, Helek 3, siman 77, page 261.)
In any event, as for the final Halacha, it is permissible to serve as Sandak many times, even for several children within the same family. (Soba Semahot, Helek 2, page 78)
There is a custom for the Sandak (and the Mohel) to immerse in a Mikveh before the circumcision. Since serving as Sandak has a siMilahr status to offering Ketoret, it should be performed in purity, just as is required for performing the service in the Beit Ha'mikdash. Additionally, the Mitzva of holding the baby during a circumcision serves as atonement for sins the Sandak may have committed involving his Brit Milah; for this reason, too, it is proper for him to immerse before the Milah, as part of the atonement process. (Soba Semahot, Helek 2, page 80)
If the Sandak and Mohel pray together in the same Minyan on the day of the circumcision, the Sandak takes precedence over the Mohel with regard to receiving an Aliya to the Torah. Thus, if only one Aliya is available, it is given to the Sandak rather than to the Mohel. (Rama, Yore Deah, siman 265:11)
Summary: It is a great Mitzva to serve as Sandak, meaning, to hold the baby during a Brit Milah. One may serve as Sandak several times even for children of the same family. It is proper for a Sandak to immerse in a Mikveh before the circumcision. The Sandak takes precedence over the Mohel in receiving an Aliya to the Torah on the day of the Brit Milah.
