There's no way to answer that. It expends what it is you are trying to adjust. A high ppm solution takes a lot more ph* or ph- to shift it.
I said its strong- obviously it's not as strong as pure sulphuric acid or anything like that. But it does work well. Just. Start with a couple drops and kelp checking your ph meter.
You can use lemons, or buy the bottled stuff. Or get powdered citric acid.
Out of the two 'natural' household choices, lemon juice or vinegar will be the two usual suspects.