For those who don't know, the Post in Royal Post is the same as in Post Office and has nothing to do with border keeps or guards...
If you hover the cursor over your gold at the top of the screen, you'll get a rollover tooltip telling you how many provinces you can personally control at 100% efficiency. It is based on your stewardship attribute. Any province above that in your demesne means a 10% penalty for all provinces...
Royal Posts (and Grand Palaces) guarantee you a minimum efficiency in the province they're built (but you'll have to edit the files on the Royal Post, they are broken in 1.01).
If you are looking for increased income and troops, your best bet is to stay at no more than 2 provinces above your maximum. 7 provinces at 80% is better than 5 at 100%, if they are all equally good. Give the rest to vassals.
There are circumstances when I personally go way above my max, using (edited) Royal Posts in my core to keep some cash going in : to try and convert heathen (+100 piety to the holder of the province, so I keep them untill they've converted, they do so quickly enough) or to anchor my border with the muslims (they tend to attack vassal counties near the border, but leave alone my own demesne, so I don't give them easy targets when I want to be left alone...)