The Life in this world is a test for the Hereafter
It is mentioned in the Qur’an:
“Do men think that they will be left alone on saying, “we believe”, And that they will not be tested?” (Al Qur’an 29:2)
There are several verses in the Qur’an which clearly specify that our Creator tests us in several different ways
**He who created Death and Life, that He may try which of you is best in deed; And He is the exalted in Might, oft-forgiving **(Al Qur’an 67:2)
And we test you by evil and by good by way of trial - to Us must ye return.”
(Al Qur’an 21:35)
This life that we lead in this world is a test for the hereafter. If we follow the commandments of the Almighty Creator and we pass the test, we shall enter Paradise i.e., which is Eternal Bliss. If we do not follow the commandments of our creator and fail the test then we shall be put into Hell.
There is a general rule that the more difficult the test, the higher the reward.
Allah (swt) tests, different people in different ways, some with health, some with disease, some with wealth, some with poverty, some with more intelligence, some with less, and depending upon the facility He gives the individual, He tests accordingly.
**“Be sure we shall test you with something of fear and hunger, some loss in goods or lives, or the fruits (of your toil), but give glad tidings to those who patiently persevere.” **(Al Qur’an 2:155)
As to the question of how many believers will enter Hell, only the Creator knows that answer for He is the Most Merciful and the Most Just.
He can be only Merciful if He so chooses and allow everyone including the evil-doers and the wrong-doers to enter Paradise… but that would not be Just to those of His creations who have faithfully followed His commands during their life in this world and have in turn been harmed by those evil-doers.