If i add a single texture as a Displacement texture on Cycles, how can i modify? i had been trying with Mix but i don't get the right number, or maybe is the wrong option?
Displacement expects a number, not a color. You can see that because the displacement's input is grey whereas a color-value is marked as yellow in the Node Tree.
So you normally convert the color to a value via a
RBG to BW Node. You then can use
Math nodes to freely manipulate the value until it fits your needs. In your case I would use a Multiply and then play with the factor a bit until satisfied.
I've used a Voronoi Texture as a stand-in for whatever texture you want to use, so you can just swap that out.
Note: I've heard that this conversion is done automatically when you connect a color-output to an value-input. Apparently this won't lead to any problems, so you can leave out the
RGB to BW node and just connect the color output to the Math input (thanks @TARDIS Maker).
You can use the same image texture as a color and bump maps at the same time. I'll show you how it works using the image below.
Set up the nodes as pictured below. Add a RGB to B&W node (Add-->Converter-->RGB to B&W). It will turn your color texture to a B&W so you may use it for bumps. To control the bumpiness size just add a Mix RGB node, set its type to Multiply and the color to white (if you're using a lower socket) or black (if you're using a upper socket). Change the Factor to control the bump size.
Here's the result:
Instead of adding the Mix RGB node you may add a Math node (Add-->Converter-->Math). Set its type to Multiply as well and play with the Value slider.
There is also the other way. Add a Bump node (Add-->Vector-->Bump) and set it up as pictured below. Increase the Strenght to make the bumpiness bigger.