What would you expect the pH of an antacid to be?

The pH of gastric acid under normal conditions is around 2 or 3.
Excess acid can reduce the pH to surprisingly low levels, sometimes right down to 1.

The purpose of antacid (despite the name) is not to render the pH of the stomach a neutral 7 (which would be just as unhealthy as a highly acidic 1), but to return it to the normal 2 or 3. Furthermore, the antacid needs to be able to do this without the patient having to consume large quantities of it.

So by consuming a small quantity of a relatively strong base (pH between 8 and 10) the pH of the stomach is reduced back to the normal levels again. pH levels above 11 would not be safe to ingest, as they could be corrosive.

Typical ingredients are magnesium carbonate, magnesium hydroxide, or aluminium hydroxide. An old and often used home remedy is to mix sodium bicarbonate in water, and drink it. This does work, but has to be used carefully, as it is water soluble and can risk raising the stomach pH above 7. If that happens, the stomach compensates by producing more acid, which is not good.