Amalickiah continues to conquer by deception rather than brute force. He has been able to keep the word of his defection and elevation to rule over the former people of Lehonti a secret from the over king. Thus the king in Nephi simply expects that this is the return of the triumphant army. He comes to meet the army with only his guards. There might have been more available army in the area, but it is most likely that the majority of them were sent with Amalickiah on this punitive expedition.
When he is met by the servants of Amalickiah, those servants prostrate themselves. This is probably the standard mode of greeting when servants approach the king. While Mormon gives us this description, the event itself was designed to continue the impression that all was well. Had it been something extraordinary, it might have put the king’s guards on heightened attention.