How much is too much? The answer to this apparently simple question is very complicated: as complicated as the number of ingredients used in skin care. Also, for a good answer, you may have to go to numbers that are much smaller than a percent (1%) and go to parts per million.
In the best of cases more is likely to be a waste of money. You are throwing away the money you spent on the active by trying to add more. For example, if you add more ascorbic acid to a serum than will possibly dissolve (as determined by the laws of nature and how water interacts with ascorbic acid) the rest will become sediment at the bottom of the tube.
Figure 1. More is just a waste of money (example: epidermal growth factor, amino acids)
In other cases, the excess of, let’s say copper, will be toxic to your skin and some cell processes will be disrupted. Your cells will become sick and then die. The visible effect on your skin will be more wrinkles, or a change in color, or a loss of fat, etc.
Figure 2. More is a VERY BAD idea. Example: copper, copper peptides, ascorbic acid.
We need to respect the actives. Just as they can benefit your skin, some of them may be deleterious at higher concentrations. If an active works through a receptor, the cells may decide to make less of that receptor to compensate for too much active coming in. This is what happens in diabetes, when too much sugar leads to resistance to the hormone insulin.