I know that during the eve of the fifteenth of Sha‘baan, Allaah The Almighty names for the Angel of Death the souls he is required to take during the following year. How can the Angel of Death seize so many souls at the same moment? Does he have assistants?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

It is proven through the Quran and Sunnah (Prophetic tradition) that the Angel of Death has assistants from among the angels.   Allaah The Almighty Says (what means): {And He is the Subjugator over His servants, and He sends over you guardian-angels until, when death comes to one of you, Our messengers take him, and they do not fail [in their duties].}[Quran 6: 61]

Commenting on this verse, Ibn Katheer mentioned that there are some angels who are appointed for this specific task. Furthermore, Ibn ‘Abbaas and others mentioned that the Angel of Death has assistants who  take the soul out of the body and when it reaches the throat, the Angel of Death seizes it.

This was also indicated in a Hadeeth (narration) reported on the authority of Abu Hurayrah in which the Prophet, sallallaahu ‘alayhi wa sallam, said that at the moment of someone's death, the angels surround him; if he is a righteous person, they say: 'O good soul, come out from this good body'. [Ahmad and others] 

The Angel of Death is able to seize many souls at the same moment because Allaah The Almighty shrinks the earth for him until it seems as if it is a vessel between his hands.  Al-Qurtubi mentioned that it was  reported on the authority of Mujaahid that the world being between the hands of the Angel of Death is similar to a vessel between the hands of a human; he takes from whatever place he wants. A similar Marfoo' Hadeeth ( i.e., with an ''elevated'' chain) was reported by Zuhayr ibn Muhammad. [Ibn Katheer in At-Tathkirah] 

The preponderant view of scholars is that Allaah The Almighty annually decrees the affairs of His creation during the Night of Al-Qadar in the month of Ramadan; not on the eve of the fifteenth of Sha‘baan.

For more details, please review Fatwa 11483.

Allaah Knows best.