The sample thickness is irrelevant as long as it is thicker than a few microns (otherwise you might see get interference effects in your spectra). The lower limit depends on the ATR crystal and the angle of incidence. Concerning the upper limit, there is no restriction, except by the fact that you must be able to clamp it. As intimate contact between ATR crystal and sample is very important. Keep in mind that oxyanions like phosphate and carbonate have very strong vibrations, which are accompanied by strong changes of the refractive index. Those will appear strongly red-shifted in the ATR spectrum, with unusual band shapes and intensities. The good news is that correction formalism are available. We discuss the related problems under https://www.researchgate.net/post/ATR_correction-why_would_you_not_use_it