I have one small fan attached on the outside of my tank, at the very top. The tank is only about 15 inches wide, but is 30 tall, and 12 deep. Some parts stay humid all the time becuase of the high plant concentration, but other get soo dry that I have actually reduced the amount of time the fan is working. So it really is up to you to find out through trial and error how often, how much, where, and how many fans you decide to use. The dynamics are gonna be different in every tank.
As an example, I have an identically built tank next to the one I mentioned above. Its full of bromeliads and vines, and is always faster to dry out, and usually less humid.
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All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
-J.R.R. Tolkien, LOTR, Fellowship of the Ring
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