Our detection threshold for citric acid is like 0.04 mM (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629

Our detection threshold for citric acid is like 0.04 mM (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3629876/), or almost 2.3*10^17 protons per 10 mL sample used in the study I linked to. Obviously you're not going to be able to taste a single proton in your mouth. But maybe only a very small fraction of those protons are actually interacting with the receptors. Figuring out the threshold on the cellular level would be complex because it would depend on the rate of diffusion of H+ through the relevant channels and the number of relevant channels.