For defense only, it somewhat depends on the province terrain. For mountains, cavalry is weakened and archers are strong. So, you'd want a lot of spears and archers to take advantage. For plains, cavalry is stronger so have plenty of them on hand. Of course, the most important thing is really the soldier numbers of the enemies, particularly the aggressive ones, who live nearby. Spy them and then run scenarios with your battle calculator to see what you would need to win. If their borders are too high for spies, look at their battle history. The most recent you can see will be 2 weeks old but it will still give you a good general idea.
Marshals near him will make him a gold mine if he start spying on them =)
