Origianally, 2000 young Anti-Nephi-Lehies (who were too young to take the pacifist vow of their parents) joined Helaman's army.
But behold, it came to pass they had many sons, who had not entered into a covenant that they would not take their weapons of war to defend themselves against their enemies; therefore they did assemble themselves together at this time, as many as were able to take up arms, and they called themselves Nephites.
And they entered into a covenant to fight for the liberty of the Nephites, yea, to protect the land unto the laying down of their lives; yea, even they covenanted that they never would give up their liberty, but they would fight in all cases to protect the Nephites and themselves from bondage.
Now behold, there were two thousand of those young men, who entered into this covenant and took their weapons of war to defend their country.
... And now it came to pass that Helaman did march at the head of his two thousand stripling soldiers. Alma 53:16-22
And later they were joined by another 60, making the total 2060.
An addition to our army ... sixty of the sons of the Ammonites who had come to join their brethren, my little band of two thousand. Alma 57:6