Al-Muddaththir 74:31

We have only appointed angels as keepers of the Fire, and We have only made their number as a test for the disbelievers[8], so that those who were given the Scripture may have certainty and the believers may increase in faith; and neither those who were given the Scripture nor the believers may have any doubts; and so that those [hypocrites] in whose hearts is sickness and the disbelievers may say, “What does Allah mean by this number?” Thus Allah causes to stray whom He wills and guides whom He wills, and none knows the soldiers of your Lord but He. This is but a reminder to mankind.

[8] To say what they want to say, so their punishment will be multiplied for them. Abu Jahl, one of the chief pagans in Makkah, made fun of the Prophet (ﷺ) when they came to know that the keepers of Hell are nineteen angels, so he ridiculed this number and said that it is an easy number to overcome.

And We have not made the keepers of the Fire except angels. And We have not made their number except as a trial for those who disbelieve - that those who were given the Scripture will be convinced and those who have believed will increase in faith and those who were given the Scripture and the believers will not doubt and that those in whose hearts is disease [i.e., hypocrisy] and the disbelievers will say, "What does Allāh intend by this as an example?" Thus does Allāh send astray whom He wills and guide whom He wills. And none knows the soldiers of your Lord except Him. And it [i.e., mention of the Fire] is not but a reminder to humanity.

31. And We have set none but angels as guardians of the Fire. And We have fixed their number (19) only as a trial for the disbelievers, in order that the people of the Scripture (Jews and Christians) may arrive at a certainty [that this Qur’ân is the truth as it agrees with their Books regarding the number (19) which is written in the Taurât (Torah) and the Injeel (Gospel)] and that the believers may increase in Faith (as this Qur’ân is the truth), and that no doubt may be left for the people of the Scripture and the believers, and that those in whose hearts is a disease (of hypocrisy) and the disbelievers may say: "What Allâh intends by this (curious) example ?" Thus Allâh leads astray whom He wills and guides whom He wills. And none can know the hosts of your Lord but He. And this (Hell) is nothing else than a (warning) reminder to mankind.

We have appointed only angels to be wardens of the Fire, and their number have We made to be a stumbling-block for those who disbelieve; that those to whom the Scripture hath been given may have certainty, and that believers may increase in faith; and that those to whom the Scripture hath been given and believers may not doubt; and that those in whose hearts there is disease, and disbelievers, may say: What meaneth Allah by this similitude? Thus Allah sendeth astray whom He will, and whom He will He guideth. None knoweth the hosts of thy Lord save Him. This is naught else than a Reminder unto mortals.