Rashi on Balak 23:10
Who counted the dust of Jacob or the number of a fourth of [or, of the
seed of] Israel? May my soul die the death of the upright and let my
end be like his."
Who can count the dust of Jacob: As the Targum [Onkelos] renders, “the children of the house of Jacob, [concerning whom it was stated,
'they shall be as many as the dust of the earth, or one] of the four
camps” - [referring to] the four divisions. Another interpretation:
The dust of Jacob-The number of mitzvoth they fulfill with dust are
innumerable: “You shall not plow with an ox and a donkey [together]”
(Deut. 22:10);“You shall not sow your field with a mixture of seeds”
(Lev. 19:19), the ashes of the red cow (19:19), the dust used for a
woman suspected of infidelity, and others similar to these. — [Mid.
Tanchuma Balak 12, Num. Rabbah 20:19]
or the number of the seed of: [The word רֹבַע denotes] their copulations; the seed which issues from sexual intercourse. — [Mid.
Tanchuma Balak 12, Num. Rabbah 20:19]
Rav Hirsch translates this pasuk as
Who then counts the dust of Jacob and the number of purely physical
animal-like births of Israel.
Rav Hirsch explains that this means that someone who attempts to consider the strength of a people by the sheer physical numbers is completely mistaken. The term Jacob means the people on a physical level. The term Israel means on the level that allowed Jacob to have his name changed to Israel by reaching the higher spiritual level. In either case, Bil'am is stating that any one who regards the strength of Bnai Yisrael on a physical level, so that he thinks that all that needs to be done is reduce that physical level, does not understand what is going on.
מי מנה עפר יעקוב וגו, You, Balak count the numerical size of this
people (CH. XXII,5 I.L.), the number of its fighters, by that
reckoning it seems too big for you, and you believe that if one could
reduce it in that direction you would be able to conquer it, you
believe that if one altogether one could get at it vially if one could
succeed in bringing a curse on its material bodily thriving: מי מנה
עפר יעקוב, who then counts what is earthly in Jacob, ומספר את רבע
ישראל, and counts the physically born numbers of Israel (- רבע in the
sam sense as in Lev. XIX,19 & XX,16 -). The prestige of other nwations
may rest on the number of the "bodies" of its population, their
increasse may form a threat to be feared by other nations, their
decrease give rise to hopes. Not so is itwith Jacob-Israel. Whether
they are small in numbers as "Jacob" or growing numerous as "Israel",
it is not that which is עפר and which increases or diminishes itself
just by animal-like breeding in which its importance, its happiness or
unhapiness consists. Not even death have they to fear, bodily deathe
cannot reach their true selves תמות נפשי וגו. I would like to die as
they do, to know that my end would be like theirs, their dying is more
blissful than our living, because they are ישרים, because they
correspond to the calling for which on the whole men are "men" and
strive for this purpose in a "straight line" without turning aside.
Summarising this first "parable" of Balaam, he had thereby to point
out to Balak the complete specific difference of Israel to all the
other nations, and to tell him that the future of this nation was
completely independent of the physical-material causes which could be
assumed to be the conditions for the growth and development of other
nations. Hence they would also be unreachable by all damaging
influences to which the welfare of other nations could be made to
succumb. But as his words made the judging of the well-being of this
nation altogether depended on the principles out of which it made its
prosperity, this speech is a true משל, one that goes beyond the
concrete single case, a sying which brings a fact, a moral, applicable
generally out of the actual example quoted.