The garrison needed depends on where the settlement is and its vulnerability to attack. Settlements on the frontline will obviously need a high quality garrison constiting of a variety of troops (although mainly infantry since they do best in siege battles), while a quiet settlement in the centre of your empire will probably only need a batch of peasants satisfactory enough to provide decent levels of public order.

Having said that, peasants are, in R:TW, the most efficient garrison troop - they're cheap and numerous and provide a higher public order than most other units (due to their larger size). This isn't the case in BI due to an attribute decreasing their affect on a settlements happiness.

As for your ten units dillema, you will need to consider several things when adding or taking away the number of units. If you are taking units away ask yourself - will they make the settlement vulnerable from invaders and/or will they decrease public order? If you are adding units, you need to query their benefits and if they will be of any use elsewhere.