John Calvin on Isaiah 10:19

19. And the remnant of the trees of his forest shall be a number. And the rest of the trees of his forest shall be few. (Margin, Heb. number.) — Eng. Ver. When he adds that the remnant of them will be a number, he employs a Hebrew idiom to express that they will be few; and thus he only confirms the former statement, that the devastation produced, after the calamity which God will bring on the Assyrians, will be so great that there will be no difficulty in counting them.

That a child may number them. He goes so far as to say, that they will be so few that those who are left of them may be counted even by a child; for children have difficulty in counting as far as three or four. Accordingly, the kingdom of the Assyrians was formerly like some vast forest; but when the trees have been hewn and thrown down, those which are left are few, and scattered at great distances from each other.