A. V. Ivanov, historian of Russian language wrote:
“Особенностью древнерусского языка по сравнению с современным было отсутствие числительных как особой части речи, которая есть в современном русском языке (речь идёт о количественных числительных). Это не значит, что не было слов, выражающих числовые понятия… Числительные до четырёх по грамматическим свойствам сближались с прилагательными, а с пяти – с существительными.”
(The peculiarity of Old Russian language in comparison with the contemporary one was the absence of numeral adjectives as a special part of the speech, which can be found in modern Russian language (the matter is about cardinal numbers). This doesn’t mean that there weren’t words to express the concept of numerals… According to grammatical rules, numerals up to four were declined as the adjectives, while numerals from five were declined as the nouns.)
In today’s language, we can see the legacy of this situation. Let’s compare the two following words: “десять” and “десяток”. Both mean “ten”. It is impossible to say “два десять”, but we freely use “два десятка” (two ten), or “четыре десятка” (four ten), because “десяток” is a substantive and “десять” a numeral.
Numerals aren’t objects, but abstract numerals and should not be regarded as a pumpkin or an apple. Professor Bogolepov wrote about the words “тысяча” (thousand), “миллион” (million), “миллиард” (billion): “The form of the system of these numbers and their use correspond to their value, thus the words “тысяча”, “миллион”, etc. represent not just a quantity, but this number as the unit of counting, therefore, as an object; it is about as easy to count thousands and millions as it is to count onions and cucumbers: one onion – one thousand, two onions – two thousand, one cucumber – one million, five cucumbers – five million, and so on.”
The word “тысяча” has a numeric value, but is it enough to relate it with numerals? Then numerals should be attributed to words such as “пара” (pair), “дюжина” (dozen), “сотня” (hundred). And what about the words “треть” (third), “четверть” (quarter), “двойка” (two), “пятёрка” (five), “пяток” (five), etc.?
And yet there are groups of words indicating vaguely a large number of something: “уйма” (lots of), “масса” (a large amount of), “пропасть” (loads of), “бездна” (a huge number of), etc.
So many questions remain.