There are literally many, many- many civilizations that existed before the arrival of Prophet Muhammad. From the western side of the world until the eastern side of the world, there are millions of people existed in this world. Yet, the Qur'an only mentioned clearly the story of prophets and their people which are mostly coming from Middle East region. The rest of the world history is leaved to the reader alone. While at the same time, there are people who lived at American Continent, Europe, Africa, India, East Asia, and Polynesia. If all those places' populations are combined and compared to the Middle Eastern region, it will be like the size of rabbit compared to the size of elephant.
For simple comparison, China population at 2 A.D. is around 59 million people. It is almost 132 times of Cordova, Spain at 1000 A.D. and 3-4 times of Saudi Arabia current population at 21th century. Remember that population boom only happens around 18th century. Did God left all these Chinese people and other people alone without any guide until the arrival of Prophet Muhammad? And if there are the believers among them, did they exist so much?
والسابقون السابقون. أولئك المقربون.
And the forerunners, the forerunners. Those are the ones brought near
[to Allah].
Quran 56:10-11
ثلة من الأولين. وقليل من الآخرين.
A [large] company of the former peoples. And a few of the later
peoples,
Quran 56:14-15
Based on the verses that have been provided at the beginning of question, the Qur'an told us indirectly, their numbers and stories are just too much. If we also count the population of Jinn before the creation of Adam, of course, the word "forerunners" here will be extremely huge. In the past, they (human and Jinn) have their own messengers, and the believers among them are the ones brought near to God. These verses are the most logical explanation about those people who came before the arrival of Prophet Muhammad. Because there is a good Hadith that told about this stories.
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَمُحَمَّدُ بْنُ بَشَّارٍ، -
وَاللَّفْظُ لاِبْنِ الْمُثَنَّى - قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ
جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي إِسْحَاقَ، عَنْ عَمْرِو بْنِ
مَيْمُونٍ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنَّا مَعَ رَسُولِ اللَّهِ صلى
الله عليه وسلم فِي قُبَّةٍ نَحْوًا مِنْ أَرْبَعِينَ رَجُلاً فَقَالَ
" أَتَرْضَوْنَ أَنْ تَكُونُوا رُبُعَ أَهْلِ الْجَنَّةِ قَالَ قُلْنَا
نَعَمْ . فَقَالَ " أَتَرْضَوْنَ أَنْ تَكُونُوا ثُلُثَ أَهْلِ
الْجَنَّةِ " فَقُلْنَا نَعَمْ . فَقَالَ " وَالَّذِي نَفْسِي
بِيَدِهِ إِنِّي لأَرْجُو أَنْ تَكُونُوا نِصْفَ أَهْلِ الْجَنَّةِ
وَذَاكَ أَنَّ الْجَنَّةَ لاَ يَدْخُلُهَا إِلاَّ نَفْسٌ مُسْلِمَةٌ
وَمَا أَنْتُمْ فِي أَهْلِ الشِّرْكِ إِلاَّ كَالشَّعْرَةِ الْبَيْضَاءِ
فِي جِلْدِ الثَّوْرِ الأَسْوَدِ أَوْ كَالشَّعْرَةِ السَّوْدَاءِ فِي
جِلْدِ الثَّوْرِ الأَحْمَرِ " .
'Abdullah (b. Mas'ud) reported: We, about forty men, were with the
Messenger of Allah (ﷺ) in a camp when he said: Aren't you pleased that
they should constitute one-fourth of the inhabitants of Paradise? He
(the narrator) said: Yes. He (the Holy Prophet) again said: Aren't you
pleased that you should constitute one-third of the inhabitants of
Paradise? They said: Yes. Upon this he again said: By Him in Whose
Hand is my life, I hope that you would constitute one-half of the
inhabitants of Paradise and the reason is that no one would be
admitted into Paradise but a believer and you are no more among the
polytheists than as a white hair on the skin of a black ox or a black
hair on the skin of a red ox.
Sahih Muslim
For addition, if we do simple calculation, the following calculation tells the number of these groups.
God's servants will be divided into three kinds 56:7; the companions of right 56:8 (Group R), the companions of left 56:9 (Group X), and the forerunners [56:10] (Group A). The composition of Group A is "large number of the former & few of the later" 56:13-14, The composition of Group R is "large number of the former & large number of the later" 56:39-40, and the composition of Group X is unknown (N). Based on the hadith, the believers among Prophet Muhammad ummah could constitute 1/4, 1/3, or 1/2 of Paradise inhabitants.
The other inhabitant could be constituted from Noah, Abraham, Moses, Jesus, or other prophets' ummah (Group 6). If word ثلة (large number-group, but not majority) can be noted as ratio r, and word قليل (small) can be noted as ratio s, then, 0 < s < r < 1/2. If ratio of Group A + R + X is (r, s) + (r, r) + (N, N') = (1, 1), then if we assume ratio r from former and later people are same, thus the ratio of Group X from former people N = 1 - 2r, and the ratio of Group X from later people N' = 1 - r - s, where N' > N. Thus, the unbelievers from later people could be larger than the former people.
If population of former people is Q, and later people is P, then total population of former people in Paradise is 2rQ and later people is (s+r)P. Hence, if 1/2 of Heaven population comes from Muhammad followers, and the ratio of Group A and R of former people is r+r = 2r, and from later ones is s+r, which 2r > s+r, but 2rQ = (s+r)P, then P > Q. In other words, despite of its short timing of ruling the Earth, total population (believers and unbelievers) of later people is bigger than former ones. And the population of unbelievers from later people could be higher than the believers if r+s < 0.5.
The conclusion is, although our generation (after Prophet Muhammad) could be the biggest believers group in the Heaven, but the population of the believers in our generation could be fewer than the unbelievers.
