the pH scale, while often taught that it runs from 1 to 14, or sometimes 0 to 14, can actually assume any value. I imagine there are physical limits but certainly negative pH or pH greater than 14 is possible, just not very common. If the concentration of protons is 10
molar, the pH would be -1.
One comment - pH, at least when measured, is an index of proton activity, not proton concentration. Its one of the few chemical parameters where activity can be measured directly.