This is something I do a lot so maybe I can help a little: It depends a lot on what you want to coat and with whatwith but the priciple is the same. The simplest example is alkyl thiols on gold. There we know that one surface gold atom can take one thiol. We need to estimate how closely packed the gold atoms are on the surface. A simple estimate is that they are touching so the distance between them is one atom diameter (0.15 nm). Gold is hexagonally close packed, so on the surface we cover 90% with atoms (https://en.wikipedia.org/wiki/Circle_packing). I Ignore a lot of details here but roughly true). Calculate the area of your nanoparticle (A = 4Pi * r^2), multiply by 0.9 and you know the number of surface atoms. Now calculate the number of atoms in your nanoparticle: density of gold = 19 g/cm3, V= 4Pi/3*r^2. Now you have the surface to bulk ratio. If it is 20% surface atoms (depends on how big your particle is) add 20% of your coating and you will have a nice monolayer. If you have other materials it may get a bit more involved but the principle is the same. Hope it helps.

You can use X-ray Photoelectron Spectroscopy(XPS) data for the calculation of shell thicknesses in core shell nanoparticles. This method can be applied by XPS analysts and does not require any numerical simulation or advanced knowledge. The suggested method for XPS analysts is simple and straightforward, allowing to apply it without the need for numerical simulation or advanced knowledge. This implies that the method is designed to be accessible and usable by analysts with basic proficiency in X-ray photoelectron spectroscopy (XPS) techniques.

Please find the attachment below for your reference. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4829121/

Actually this is not possible. The shell thickness differs from one particle to another particle. Using XPS spectrum, you can deduce an average approximate value.