John Gill Commentary Numbers 26

And it came to pass after the plague
Related in the preceding chapter; how long after is not certain, perhaps before the war with Midian, exhorted to in the latter part of the preceding chapter, and of which an account is given, (Numbers 31:1–54) :

that the Lord spoke to Moses ;
out of the tabernacle, or out of the cloud:

and to Eleazar the son of Aaron the priest ;
the Lord had been used to speak to Moses and to Aaron; but now Aaron being dead, and Eleazar his son succeeding him in the priesthood, is joined with Moses, and the order here given is directed to them both:

saying :
as follows.

Take the sum of all the congregation of the children of Israel ,
&c.] Excepting the Levites, who were to be numbered by themselves, and at a different age; this sum was to be taken, that it might appear that all of the old generation that came out of Egypt, of the age at which this sum was taken, were now dead, excepting two, as the Lord had threatened.

And partly that as they were now about to enter the land of Canaan, it might be divided to them according to their number; as well as to show the faithfulness of God to his word and promise, that he would multiply and make them fruitful, notwithstanding all their provoking sins and transgressions:

from twenty years old and upwards, throughout their father's house ;
all of that age in every tribe, house, and family:

all that are able to go to war in Israel ;
for which they must prepare, being about to enter the land of Canaan, and dispossess and drive out the inhabitants of it.

And Moses and Eleazar the priest spoke with them
With the children of Israel, with the heads of them, their chief and principal, to assist in taking the number of the people; as when they were numbered thirty years ago, when a prince out of each tribe was taken to be with Aaron and Moses in doing that business; but those princes were now all dead, and another race succeeded, who were now employed in this service; so the Targum of Jonathan says, they spoke with the rulers, and ordered them to number them:

in the plains of Moab, by Jordan, [near] Jericho :
or of Jericho, as the same Targum, on the other side of Jordan to that on which Jericho stood; for as yet the children of Israel had not passed that river, nor entered into the land of Canaan, in which Jericho was, but they were now opposite it; (See Gill on Numbers 22:1):

saying ;
as follows.

[Take the sum of the people], from twenty years old and upward ,
&c.] At the same age at which the sum was taken before, (Numbers 1:3) so that there could not be one that was more than sixty years of age, of all those that went into the land of Canaan, except Joshua and Caleb, and besides some few in the tribe of Levi, which did not come into either of these musters.

and the Lord commanded Moses, and the children of Israel, which went
forth out of the land of Egypt ;
as Moses had a command to number the people before, so he had now. The sin of David was, that he numbered the people when he had no command for it; Moses, when he brought the people out of Egypt, had them committed to him by number; and now being about to die, he delivers them up as it were by number again, as Jarchi observes.

Reuben, the eldest son of Israel
the number of his tribe is taken first on that account; there were four families that descended from him, the Hanochite, Palluite, Hesronite, and Carmite, and the number of men from twenty years old and upwards, fit for war, were 43,730; so that, since the last numbering, this tribe was decreased 2770; which may in part be accounted for by the families of Dathan and Abiram being cut off for their rebellion, who were this tribe, and whose affair is briefly related in the following verses, and it may be to point out this unto us.