I`m trying to apply it for each texture unit but it seems only to affect the second unit or just have a general effect, like the scaling is done after the combining of the units A call like-> glTexEnvi(GL_TEXTURE_ENV, GL_RGB_SCALE_ARB,2); Can be done per texture unit or globally ?

it's is a per-tex unit effect, and it is applyed after the texture unit maths...

So if you had first texunit is modulate, second is add, eg, (t1*v)+t2, if you set the scale of the second to 4, the equation becomes ((t1*v)+t2)*4. If it was the first, then the equation is (t1*v)*4+t2

etc.