How much do edge chips on a solar cell reduce its maximum amperage?

If the conductors in or to or from the cell are broken, it will reduce amps by one cell's worth. If possible, measure the output of the individual cell before connecting it. I don't know enough about the details to know for sure but your math suggests you are not building a 100 W panel, you are building a 88 W panel. As far as affecting your output too much, that depends on what you are doing with the output. As you probably know, something rated for 20 volts will operate at 20 +/- some number of volts. How much +/- depends on the details. sounds like a good project! Good luck