Which is the measurement unit of the Intensity? It is the amplitude of a complex number so can I say it is a pure number? and what about the coherence ?(as it represents correlation it is a pure number too)

Intensity is the uncalibrated measure of engergy which returns to the sensor. It is the squared measure of amplitude (A*A = I). It consists of the real (i) and imaginary (q) part of the complex radar signal but intensity itself is no longer complex. Here are some good explanations of that: https://earth.esa.int/handbooks/asar/CNTR5-2.html#eph.asar.gloss.radsar:COMPLEX_NUM

About coherence, please see here: Some explanations about concepts of fringes of interferogram and coherence

The links are quite clear, so in the end they have no physical measurement unit, unless we do some conversion , am I wrong?

yes, intensity has no unit. Unless the calibrated signal is not converted into log scale [db] backscatter is without unit.

perfect ! thanks!

Just to clarify, intensity does not have any units but the radar cross-section (i.e. backscatter) of a distributed target (e.g. a forest) does have units of m^2/m^2 (i.e. meter squared divided by meter squared). So backscatter is dimensionless but it does have units. Since backscatter is using given in dB (10*LOG10) the m^2/m^2 is not written - for example the backscatter of a forest would be written as -10.2dB.