Fortress to Province ratio

[das123]

Forts seem to serve two purposes. 1: They increase the production amount that can be taken from the surrounding provinces; and 2: They slow down attacks on your territory.

I tend to build forts behind my front-line so that each fort is two provinces apart to maximise the production. My front-line is usually forts next to each other to slow down attackers.

The biggest problem I have is that by the middle game I usually only have 1 real army with all my attacking forces in it (around 8 commanders). If the AI attacks my territory with two or three armies they can basically take the whole lot in short time and I am just playing catch-up but never make any real in-roads. Question I have is how many armies do you guys build in relation to the number of provinces you own? And how much do you rely on province defence?

Also, by having just one army, when it gets wiped (and it always does) I am in a lot of trouble. That's why I try to build forts instead. How do you guys cater for this?