DOES ONE BLESS ON EVERY CUP? [Berachah:Rishonah:repeating]
Ameimar blessed on every cup, because he reconsidered (to drink more) each time. Mar Zutra blessed on his first cup and the cup of Birkas ha'Mazon, like Talmidei Rav (who accepted Rav Yeva's words). Rav Ashi blessed only on his first cup.
Chulin 107b - Rav: A waiter blesses before every cup of wine he receives, but not on every slice of bread.
R. Yochanan: He blesses before every slice of bread.
Rav Papa: They do not argue. He need not bless for every slice only when an important person is present.
Rif: The Halachah follows Mar Zutra. When one reconsidered, the Halachah follows Ameimar.
Ran (DH Itsar): The Rif explains that Talmidei Rav hold that declared intent to say Birkas ha'Mazon (and all the more so saying it) forbids eating, i.e. without a new Berachah Rishonah. Others say that it totally forbids eating before a Berachah Acharonah.
Rambam (Hilchos Berachos 4:7,8): If one decided in his heart not to eat or drink more and later reconsidered, he must bless.
Source (Kesef Mishneh): A waiter blesses on every cup, because he did not expect it. He blesses on every slice of bread, unless there is an important person there (for then he is assured of getting enough to eat). This shows that eating and drinking are the same regarding reconsidering.
Ra'avad: Indeed, deciding not to drink more obligates a new Berachah. However, only Mayim Acharonim or sleep obligates a new Berachah before eating more.
Source (Kesef Mishneh ibid.): The Ra'avad learns from Berachos 42B, which concludes that only Mayim Acharonim makes a Hefsek. To rebut the Rambam's source, the Ra'avad must say that a waiter is different, for he is not Kavu'a to eat. Presumably, the Rambam had a different text in Berachos.
Rambam (ibid.): Declared intent to bless or say Kidush forbids drinking more before doing so. If they will continue drinking, even though it is forbidden, they must bless Borei Peri ha'Gafen first.
Rosh (10:10): Declared intent to bless forbids drinking without a new Berachah Rishonah. They need not say Birkas ha'Mazon first, unlike the Rashbam.
In Berachos (Sof 6:31) the Rosh rules like the Rashbam, and equates eating with drinking.
Shulchan Aruch (OC 174:5): One Berachah suffices for all that one will drink during a meal, unless he did not intend to drink more and reconsidered.
Rema: If one comes to a meal he blesses on each cup he is given. Each time he did not expect to drink more, and reconsidered.
Taz (6): Similarly, if one bought bread and ate it, and then decided to buy and eat more, a new Berachah is needed. However, one who blesses on bread in his house and later decides to eat more need not bless again, for this is normal.
Kaf ha'Chayim (24): Eliyahu Rabah and Pri Megadim require a new Berachah even for bread in his house, unlike the Taz. It is a Safek whether or not to bless, so we are lenient.
Kaf ha'Chayim (27): Nowadays, the custom is that when one joins people who are drinking, many give to him. Therefore, one blessing suffices, like for a waiter regarding bread in a place where there is an important person.
Mishnah Berurah (19): One who comes to (joins) the meal must bless on each cup, but those who were invited need not.
Rema (ibid.): When people do not know whether the cup of Nisu'in will be finished before they get it, everyone who gets to drink from it must bless on it (they were not Yotzei by answering to the Berachah).
The Shulchan Aruch (OC 190:5) says similarly regarding the cup of Birkas ha'Mazon.
Magen Avraham (7): If the one who blessed intended to be Motzi the others, and they intended to be Yotzei, they need not bless.
Kaf ha'Chayim (28): The Taz argues, but most Poskim hold like the Magen Avraham.