Ideally, you will find a vitamin C serum with a percentage of between 10 and 20% L-ascorbic acid.
This is considered by most to be the sweet spot, and it’s specifically L-ascorbic acid that you want to look for with this concentration range.
That also means you want to be sure that this ingredient is listed as one of the first on the label, as it’s arguably the most important in any good vitamin C serum.