I do make my own liquid Stevia, 1/8th tsp pure white Stevia extract (health food stores are your best best) is approximately equal to 1C sweetness with some variation on brands...I take it from there.

E.g., if I want 1 Tsp of Liquid Stevia (homemade) to be about 1C of sweetness, I use 1 Tsp of the extract and 8 Tsp of filtered or distilled water and stir them up in a little bottle. Bingo.

For the commercial liquid Stevia(s), you can usually go on Serving Size, which is almost always equivalent to 2 Tsp of sweetness (like sweetener packets). So if it says 1 drop in your coffee, that 1 drop is about 2 Tsp sweetness, so 1 C or thereabouts would be 48 drops...you can see why I'd rather blend my own, it's both cheaper and easier to measure for cooking.

With ANY stevia mixture, however, you have to go by taste and start low, working your way up. Stevia is fab, but go too much and that aftertaste will get you.

I like it best in blends. Two or more sweeteners, you get to use less because the sweetness is greater than the sum of its parts ("synergy", a word you will see in your research).