I was thinking about the plague of darkness, where we find that our sages tell us that four-fifths, or 49/50 or 499/500 died amongst the Jewish people. To understand this on the surface level would have us calculate the Jewish people to be five, fifty or five hundred times 600,000 before the plague of darkness. The very fact that this number is so fantastic hints that chazal here are not teaching us something literal, but instead, this statement is included in the concept that they would hide deeper concepts in fantastic statements, in order to repel the uninitiated, and teach a deep lesson to those who have the key.
To understand what could perhaps be the true depth of chazal in this instance, we need to look at the numbers chazal use, as the numbers are generally the key to unlocking what chazal were actually referring to.
It is remarkable that at the point of the plague of darkness, we have passed the four fifths point of the ten plagues. Being that the plagues correspond to the ten sefiros (as do all groups of ten), we can see that the concept of four fifths means that we have passed Binah and are now in Chochmah, the ninth of the sefiros when we count from the bottom up.
If we look at the numbers 49 vs. 50, we see the concept of the seven times seven lower sefiros (as we count from Pesach to Shavuos), and the fiftieth level, which is Binah.
If we look at the number 499 vs. 500, we have the concept of צבאות, whose gematriah is 499. This concept always refers to the two legs of Netsach and Hod, which are the third and fourth from the bottom of the עץ החיים.
Thus, while we would initially perceive an ascension in the numbers 4/5, 49/50, 499/500, we are actually looking at a descending point in the sefiros.
I believe that chazal are hinting to a very deep concept here, which is spoken about by the Izhbitzer. There is a process of birur, purification, through which the Jewish people as a whole, as well as every individual, comes to the realization that Hashem is all there is, that everything is truly Him. During this process, the dross of physicality is slowly burned away, and the individual or nation recognizes that all the externals are nothing more than illusion.
During the process of the plagues, the Jewish people were being purified through this process so they could merit to leave Egypt. Each מכה had the effect of burning away another layer of externality, until the core was revealed.
As one rises through the sefiros, one finds the more rarefied essence which is at their core. In the levels of Soul, the aspect of Nefesh corresponds to Malchus, the lowest of the sefiros. Ruach corresponds to the next six; Neshama corresponds to Binah, the eighth. The makifin levels of Chaya and Yechida correspond to the last two sefiros, Chochmah and Keser, which are levels of transcendent unity.
It could be said that chazal are hinting to this idea when they say that 'four-fifths of klal yisrael died during the plague of darkness.' This could mean that the lower aspects of reality were removed, and the Jewish people had access to their higher selves, the unifying aspects of their chayah and yechida.
On another level (because everything is relative), the forty nine levels represent malchus and the next six, the seven lower sefiros (multiplied by seven). The fiftieth represents the level of Binah, which corresponds to Neshama. Here, too, we can say that there was a removal of the lower, outer aspects of the Jewish people, such that the Neshama was revealed.
On still another level, the 499 represents the aspect of צבאות, which corresponds to the concept that the Jewish people (or Nevi'im, who access prophecy through netsach and hod) are but 'hosts' for the Divine that 'rides' upon them. When the outer aspect of the 'host' is stripped away, the Divine within is revealed, which is the 500th aspect (or תתקצ"ט).
The point is that chazal are showing us that the purpose of the plagues in general, and the plague of darkness, in particular, was to strip away the layers of darkness surrounding the soul-essence of the Jewish people. The four-fifths, or 49/50, or 499/500 of the Jewish people that died here could very well not have been a percentage of the Jewish people as a whole, but rather, a percentage of each individual as their outer 'garment' of physicality was removed, and their soul essence was revealed.
This is what is to happen for each one of us as Moshiach arrives and we have the corresponding experiences to the sefiros. We will iy'H soon see Hashem revealed in each and every human being until the moment when we will realize that there really is no other besides for Him. ביום ההוא יהיה ה' אחד ושמו אחד. May it be soon. Amen.