An even earlier statistics is found in 1 Samuel 11:8
When Saul mustered them at Bezek, the men of Israel numbered three hundred thousand and those of Judah thirty thousand.
One reason for mentioning Judah separately was the fact that Judah and Philistine shared borders.
Another reason has to do the prophecy in Genesis 49:10
The scepter will not depart from Judah, nor the ruler's staff from between his feet, until he to whom it belongs shall come and the obedience of the nations shall be his.
From the beginning of the kingdom, they were interested in Judah's contributions to the kingdom. They knew early on that a person from Judah would be king.
1 Samuel 16:13
So Samuel took the horn of oil and anointed him [David] in the presence of his brothers, and from that day on the Spirit of the LORD came powerfully upon David. Samuel then went to Ramah.