Does Rashi change the number of miracles?

Summary: No, he doesn't. But I like the methodology.

Post: From DovBear:

Changed Midrash
When Yitzchak brings Rivka into his tent, he's comforted. Rashi tells us the comfort came from the fact that three miracles that Sara enjoyed reappeared. The Midrash, however, says there were four miracles. 

I must say that I like the methodology. This might just be showing that Rashi sometimes modifies his sources, and so, does not consider them sacrosanct. But in general, it is a good path to take when learning Rashi, to look up every since one of his sources and consider what Rashi keeps the same and what he changes. Based on the dibur hamatchil, is Rashi commenting on the same thing the midrash is, or is he using the midrash to his own ends? What does he change in language or content, and what does that tell us about his purpose in this particular comment, and in general. It is a good approach, even though I think in this case some of the details are incorrect.

Rashi comments on a pasuk in parashat Chayei Sarah:

67. And Isaac brought her to the tent of Sarah his mother, and he took Rebecca, and she became his wife, and he loved her. And Isaac was comforted for [the loss of] his mother.		סז. וַיְבִאֶהָ יִצְחָק הָאֹהֱלָה שָׂרָה אִמּוֹ וַיִּקַּח אֶת רִבְקָה וַתְּהִי לוֹ לְאִשָּׁה וַיֶּאֱהָבֶהָ וַיִּנָּחֵם יִצְחָק אַחֲרֵי אִמּוֹ:
to the tent of Sarah his mother: He brought her to the tent, and behold, she was Sarah his mother; i.e., she became the likeness of Sarah his mother, for as long as Sarah was alive, a candle burned from one Sabbath eve to the next, a blessing was found in the dough, and a cloud was attached to the tent. When she died, these things ceased, and when Rebecca arrived, they resumed (Gen. Rabbah 60:16).		האהלה שרה אמו: ויביאה האהלה ונעשית דוגמת שרה אמו, כלומר והרי היא שרה אמו, שכל זמן ששרה קיימת היה נר דלוק מערב שבת לערב שבת, וברכה מצויה בעיסה, וענן קשור על האהל, ומשמתה פסקו, וכשבאת רבקה חזרו:

If we look to Bereishit Rabba, we see the following:

טז [הברכות באהלה של שרה ורבקה]ו

ויביאה יצחק האהלה שרה אמו

כל ימים שהיתה שרה קיימת, היה ענן קשור על פתח אהלה,כיון שמתה, פסק אותו ענן, וכיון שבאת רבקה, חזר אותו ענן.

כל ימים שהיתה שרה קיימת, היו דלתות פתוחות לרוחה, וכיון שמתה שרה, פסקה אותה הרוחה, וכיון שבאת רבקה חזרה אותה הרוחה.

וכל ימים שהיתה שרה קיימת, היה ברכה משולחת בעיסה, וכיון שמתה שרה, פסקה אותה הברכה, כיון שבאת רבקה חזרה.

כל ימים שהיתה שרה קיימת, היה נר דולק מלילי שבת ועד לילי שבת, וכיון שמתה פסק אותו הנר, וכיון שבאת רבקה חזר.

וכיון שראה אותה שהיא עושה כמעשה אמו קוצה חלתה בטהרה וקוצה עיסתה בטהרה מיד,

ויביאה יצחק האהלה. 

Though we should not discount the possibility that Rashi has a different midrashic source, as well. For example, in Midrash Aggadah we see a parallel.
האהלה שרה אמו. שמצאה כשרה כאמו•
ואמרו חזל כי על שרה שהיה ענן קשור על אוהלה׳ וכשהיתה מדלקת נירות
בערב שבת, היו הנרות דולקות עד מוצאי שבת, וכן רבקה׳ וכסה הענן לאהל שרה
כשמתה שרה נסתלק הענן וכשבאתה רבקה חזר הענן, וכשם שהיתה שרה זהירה
בשלשה מצות שהאשה חייבת בהם׳ נדה וחלה והדלקת הנר, כך היתה רבקה זהירה

where the blessing in the dough is omitted. See also Yalkut Shimoni, which echoes Midrash Rabba, though this was not likely Rashi's source.

Turning to Bereishit Rabba, it is not the case that there are four miracles listed. Rather, just as Rashi puts it, there are only three miracles:

1. The cloud attached to the tent
2. A blessing found in the dough
3. A candle burning from one erev Shabbat to the next

However, in addition to these three, there is an additional one, or two:

1. The doors were wide open to guests, as she was also extremely hospitable
2. She separated her challah and her dough when in a state of ritual purity, rather than as a menstruant

In my reading, the Midrash Aggada (cited above) relates these miracles as rewards for her righteous actions, which makes sense, and adds the very act of lighting Shabbos candles, which was implicit in Midrash Rabba, as another action.

But Etz Yosef, a commentator of Midrash Rabba, sees only five aspects, not six, and relates it to the extra ה of האהלה. And so the total number would be five or six, rather than necessarily four. And the fourth thing was not a miracle, but rather a meritorious action.

Now, Rashi does seem to omit these extra elements. Or perhaps he does not, but only refers to them obliquely, in stating ויביאה האהלה ונעשית דוגמת שרה אמו, כלומר והרי היא שרה אמו. Either that or he omits them, and considers it more peshat-like midrash to focus on overt Divine signs that she was the equal of his mother, rather than focusing on the various meritorious actions which would have incurred this Divine favor.