John Gill Commentary Isaiah 7:21

"And it shall come to pass in that day, that a man shall keep alive a young cow, and two sheep;" — Isaiah 7:21 (ASV)

And it shall come to pass in that day
Not in the days of Hezekiah, after the destruction of Sennacherib's army, when there followed great fruitfulness and plenty, (Isaiah 37:30Isaiah 37:31) as Kimchi and Jarchi interpret it; but in the days of Nebuchadnezzar, after the destruction of Jerusalem, when some poor men were left in the land to till it, (Jeremiah 39:10) for of these, and not of rich men, are the following words to be understood:
[that] a man shall nourish a young cow and two sheep ;
this seems to denote both the scarcity of men and cattle, through the ravages of the army of the Chaldeans; that there should not be large herds and flocks, only a single cow, and two or three sheep; and yet men should be so few, and families so thin, that these would be sufficient to support them comfortably.