What is the amplitude of an EM wave?

[iScience]

i always hear about the frequency/wavelength, and the speed of the wave, but i never heard about the actual amplitude of the wave itself? how would one derive this? I'm not referring to intensity... intensity deals with the number of photons being captured by a detector... I'm referring to the actual amplitude of the EM wave itself. if it truly is a wave, it MUST have an amplitude right? what is it? how do i derive it?

 

[Redbelly98]

Page 11 of this link gives the relation between E-field amplitude $E_0$ and intensity (power per area) $I$ as

$$I = \frac{E_0^2}{2 \ c \ \mu_0}$$

Edit added:
The derivation comes from writing the energy density in terms of E and B (as given in any decent E-M textbook), and then using

$$\text{Intensity} = \text{Energy density} \cdot c$$