I don't think the public order bonus provided by garrison troops has anything to do with 'distance to capital', i'm pretty sure it is dependent ONLY on:

number of soldiers/population

I do not recommend using more than 5 garrison troops (if each unit has an upkeep of 100) at all. The only reason why you would want a large garrison is to maintain a Very High tax rate. The difference between low and very high tax rate is usually roughly equal to 500 denarii. If you use more than 5 troops you are spending more than you are gaining, which is not a good idea.