The pH of gastric acid is 1.5 to 3.5 in the human stomach lumen, the acidity being maintained by the proton pump H+/K+ ATPase.
Lemons are extremely acidic. Any chemical with a pH less than 7 is considered acidic. Lemon juice has a pH around 2.0, ranging between 2 to 3. To put that in perspective, the pH of battery acid (sulfuric acid) is 1.0, while the pH of an apple is about 3.0.