FBO's driver implementation - use more than four texture in a fragment program

When I use a 3d texture of two slices and I bind these two slices
to two different fbos will be used only the texture unit to which the
texture is originally bound or a different texture
units for every single slice? In other words: do I need a texture unit
for each fbo?
many thanks

PS I have a 6800gt with 77.72 Nvidia’s driver

I don’t understand.

FBO’s don’t use texture units. They’re not sources of texture data; they’re destinations.