Why don't we use plural "TOVOT" in New Years greeting for ketivah ve-hatimah (writing and sealing) tovot?
In the Mishneh Torah, Hilkhot Teshuvah 3:6, it says
ו[ג] ... שוקלין עוונות כל אחד ואחד מבאי העולם עם זכייותיו ביום טוב של ראש השנה: מי שנמצא צדיק, נחתם לחיים; ומי שנמצא רשע, נחתם למיתה. והבינוני, תולין לו עד יום הכיפורים: אם עשה תשובה, נחתם לחיים; ואם לאו, נחתם למיתה.
... the sins of every inhabitant of the world together with his merits are weighed on the festival of Rosh HaShanah. If one is found righteous, his [verdict] is sealed for life. If one is found wicked, his [verdict] is sealed for death. A Beinoni's verdict remains tentative until Yom Kippur. If he repents, his [verdict] is sealed for life. If not, his [verdict] is sealed for death.
There are three options:
- Being [inscribed and] sealed for life on Rosh ha-Shanah.
- Being [inscribed and] sealed for death on Rosh ha-Shanah.
- Being [inscribed but] not sealed [for life or death] on Rosh ha-Shanah, and [re-inscribed, if needed, and] sealed on Yom Kippur.
The blessing is for the first option to happen. "Tovah" is in the singular, because the inscribing and sealing of good verdict that we're hoping for is done in one action. (We're hoping for the verdict to be sealed already on Rosh ha-Shanah, not to be inscribed for life on Rosh ha-Shanah, and have to wait till Yom Kippur to have it sealed.) In fact, the Mishneh Torah only mentions one action ever being done (i.e. the sealing of the verdict).