As we saw with the incident with Gideon and Noah atop the tower, this tower was able to oversee Lamanite lands. Therefore, Limhi is warned of the approach of this army. With the Lamanite king at the head of it, we may assume that it was both large and that it made no attempt at hiding its approach. In Mesoamerican warfare the visual intimidation of the numbers and their regalia was a significant aspect of the battle, and we have no reason to assume that this occasion was different.
Unlike Noah, who was on the tower by coincidence, Limhi is on the tower precisely because of the recent hostilities between the two city/states. While under a treaty, he was nevertheless wary of another attack. Indeed, he sees this one developing, and uses the strategem of ambush to overtake the Lamanite army. We may suppose by Limhi's use of this tactic that he had significantly fewer fighting men in his army than does the Lamanite king. Had he a comparable army, honor and the canons of warfare might have dictated that he fight an open battle. Indeed, verse 11 confirms that his army is "not half so numerous as the Lamanites." Limhi sees his people in imminent danger, and uses the ambush to even the odds.