[robin]

I write pH = -log₁₀[H⁺] , but the two equations are the same really.

Taking RobJims's 38% by weight HCl means 38g of HCl in 100 ml of solution.

38g of HCl = 1.04 mol in 100 ml of solution = 10.4 M

If [H⁺] = 10.4 M, then pH = -log₁₀[10.4] = -1.0

So the highest possible pH of HCl solution would be -1.0

It's not like that in your stomach though, you wouldn't be in the land of the living.