Standing over Hell are MIGHTY and STERN angels who never disobey Allah subhaanahu wa ta’ala Who has created them. They do whatever He commands them, as Allah subhaanahu wa ta’ala says:

“O you who believe! Save yourselves and your families from a Fire whose fuel is men and stones, over which are [appointed] angels stern [and] severe, who flinch not [from executing] the Commands they receive from Allah, but do [precisely what] they are commanded.” (66:6)

Their number is nineteen as Allah (swt) says:
“Soon I will cast him into Hellfire. And what will explain to you what Hellfire is? Naught does it permit to endure, and naught does it leave alone! Darkening and changing the color of man! Over it are nineteen [angels as guardians and keepers of Hell]” (74:26-30)

This number was a challenge to the kuffaar who thought that they would easily overcome such a small number, but they did not realize that one of these angels has the strength to confront the whole of mankind on his own. For this reason, Allah (swt) said in the next aayah:

“And We have set none but angels as guardians of the Fire, and We have fixed their numbers [19] only as a trial for the disbelievers..” (74:31)

Ibn Rajab said, “What is known and well-established among the earlier and later generations (as-Salaf wal-Khalaf) is that the trial came about when the number of angels was mentioned and the kuffaar thought that it was possible to kill them. They thought that they would be able to fight and resist them. They did not know that humankind in its entirety would not be able to resist even ONE of them.” (at-Takhweef min an-Naar, p.174)