“Let Reuben live, and not die;
And let his men be few (literally, ‘may his men be a number').”
The first to be dealt with is the firstborn Reuben. Reuben's future was destined to be that they would wither as a nation, but would survive. As Jacob had said in his dying blessing, unstable as water he would not excel (Genesis 49:4). As Reuben had not been executed for his misdeed, the tribe were not doomed to final execution because of his having taken his father's concubine - Genesis 35:22, (as the law now required - Leviticus 20:11), but nevertheless they would suffer a lesser penalty in that they would not be fruitful (compare Leviticus 20:20-21). They would eventually become a depleted tribe. Moses recognised the inevitable divine consequence of Reuben's behaviour, and that the mills of God grind slowly. He may also have seen as significant that they settled by request in land which Moab and Ammon still claimed as their own (Judges 11:13), and were therefore especially vulnerable once those nations grew strong.
“May his men be a number.” This is usually seen as indicating that they would be few in number. Compare ‘men of number' in Genesis 34:30, that is, easily counted. But it might mean ‘may they be numerous', although the use in Genesis 34 is against it.