There is no mention anywhere that there were ever 84,000 sūtras as such, but there are many mentions that the Buddha gave 84,000 teachings (also called the “84,000 elements of the teachings” or “84,000 dharmaskandhas,” (Tib. chos kyi phung po brgyad khri bzhi stong). The assumption is that all 84,000 teachings are covered by the sūtras as a whole.
For example, if you look through a sūtra like The Jewel Cloud, Toh 231, there are teachings on all ten of the transcendent perfections, each of which is divided into ten, or sometimes many more, topics, some of which are further subdivided into more points. So this is one text / sūtra that probably contains thousands of points of doctrine. I doubt that anyone has ever counted and catalogued each point in all texts, and checked all the duplicates across texts, to come up with the figure 84000; if they have, they don’t seem to have left a record! So people may ask, why 84,000?
The most common explanation is that the Buddha gave (at least) one teaching to counteract each one of the 84,000 different obscurations and afflictions that beings have come up with.
In one way, the figure 84,000 can just be taken as an archetypical large number. It occurs in many sūtras with just that function, numbering all sorts of things like parasols, bodhisattvas, units of measure, chariots, soldiers, palm trees, and so on—not just teachings or afflictions.
However, there are also more detailed explanations that explain how the figure of 84,000 came to be, as an enumeration of all the subtle variations of the kleśas. Most are based on there being four groups each of 21,000—the four subdivisions being, most commonly, for attachment, aversion, ignorance, and the combination of all three, respectively. In other explanations the four divisions are in terms of the category of teaching, i.e. the antidote, into sūtra, vinaya, abidharma, and all three together. Although most explanations don’t go down to a more detailed level, there are also various different ways in which the 21,000 figure is further subdivided.
To thicken the plot, another explanation in the Pali Theragatha has Ānanda saying that he received 82,000 teachings from the Buddha and 2,000 from his senior monks.
So the short answer is that the 84,000 teachings the Buddha gave can be seen either as just a very large number of points that he made; or it can be taken more literally to mean the antidotes that he provided to the 84,000 different obscurations of beings; but that in both cases a large number of these individual points or antidotes are considered to be contained in each text. That this is indeed the case can be seen by reading any of the sūtras, particularly the longer ones.