I had mentioned two units of peasants as being enough in most cases. One spearmen and two archers unts are fine too since that gives 20 extra men but I find that archers are not really suitable for garrison purpose since their maintenance cost is rather high (it exceeds the cost of an unit of X-bows and on a man for man basis spearmen and peasants).
I also pointed out that some provinces were more likely to go rebel. I would say if you want to set the tax rate to very high, you need (in addition to border forts):
- Portugal: 800 men;
- Livonia: 600 men;
- Lithuania: 600 men;
- Scotland: 400 men.
If you loose the connection with your king, you will probably not need any extra troop and can just diminish the tax rate until the connection is restored.
I also always keep 400 men on any island I have since there is always the risk that you will lose the connection between an island and your king (enemy ship, tempest).
Bear in mind that the province religion also has an impact on the loyalty of your provinces. Before invading an unruly province, it's always a good idea to send a bishop spend a few turns there if you have the opportunty. The piety of your governor can also cause unrest if nhis religion is not the same as the one of the locals (for those provinces 0 peity with high dread gives the best results).
Finally, remeber that a drop in loyalty can be triggered by the presence of an enemy spy and that you need border forts, spies and assassins to get rid of them (but I have never experienced any spy-rush by the AI).
Hope it helps,
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