We are up to Maseches Menachos, chapter 13, mishnah 2, which deals with a person who vows to bring a minchah and with the way his vow is fulfilled.
The basic case - "Harei alai minchah" or "Harei alai min haminchah":
In both of these cases we are dealing with an individual offering: "minchah" - an obligation to bring a minchah, "min haminchah" - an obligation to bring a particular type from among the menachos. Here too, however, the one who vows does not specify which type he means, and therefore "yavi echad" - he brings only one minchah. In line with the dispute in the previous mishnah, whether it must specifically be fine flour or whether any type will do, the halachah is that he may bring any type of minchah, as long as he brings one. There is no great novelty here; this is the basic case that sets the stage for what follows.
"Harei alai menachos" or "Harei alai min hamenachos":
Here the one who vows used the plural: "Harei alai menachos" - an obligation to bring menachos, or "Harei alai min hamenachos" - an obligation to bring a particular type of menachos, in the plural. The law is "yavi shtayim" - he brings two. Here too he may bring whichever type he wishes: he did use language indicating a particular type, but he never specified which type, and he had no particular intention in mind, so he may bring any of the five types according to the halachah, or according to the opinion that disagrees. And how many does he bring? Two menachos, since he used the plural, and we assume that he meant the minimum implied by that expression.
"Peirashti v'eini yodea mah peirashti":
Here the one who vows claims: I did indeed have a particular type of minchah in mind when I obligated myself, but I do not remember which of the five types I intended - the minchah of fine flour, the machavas, the marcheshes, the rekikin or the challos. The law is "yavi chamishtan" - he must bring all five types, since he obligated himself in a specific minchah and does not remember which it was, and in this way he makes certain that he has fulfilled his obligation.
With each one of the five he makes a stipulation and says: if this is the minchah I obligated myself to bring, let it fulfill my obligation, and if not, I am hereby bringing it now as a voluntary offering.
"Peirashti minchah shel esronim v'eini yodea mah peirashti":
Here the one who vows says: I obligated myself to bring one minchah, but at the time I obligated myself I had in mind a minchah consisting of a number of esronim of fine flour, and I do not remember how many measures there were. The law is "yavi minchah shel shishim issaron" - he must bring the largest possible minchah, which is one of sixty esronim, since there is no minchah larger than that; more than sixty is impossible, because then it becomes two menachos, and he certainly did not obligate himself in more than sixty.
And if he actually had in mind, for example, thirty esronim, and does not remember, it turns out that half of the minchah is voluntary fine flour and half is obligatory fine flour, and the two become mixed together. The tanna kamma holds that this is not a problem: one may mix into a single minchah flour that is obligatory together with flour that is not obligatory, and therefore this is how he should proceed.
The view of Rebbi:
Rabbi disagrees and holds that this is impossible: one may not combine in a single minchah flour that is obligatory with flour that is not obligatory. And since he is not permitted to do so, he is forced to bring separate menachos - a minchah of one issaron, in case that was his obligation; a minchah of two esronim, in case that was his obligation; a minchah of three, and so on up to sixty. In this way he will know with certainty that he brought a minchah in the amount he obligated himself to bring, even though he no longer remembers how much it was, and that minchah will be offered as his obligation while the rest will be voluntary offerings.
Working out the arithmetic, one plus two plus three, and so on up to sixty, comes to a total of 1,830. It emerges that he must bring 1,830 esronim of flour, divided into sixty menachos as described.
The halachah: The halachah does not follow Rabbi. Therefore, a person who obligated himself to bring a minchah of a certain number of esronim and does not remember how many he had in mind brings a single minchah of sixty esronim, the largest amount possible, and he has in mind that the quantity of flour within it that fulfills his obligation should count toward his obligation, while the rest will be a voluntary offering.