Probably do the same thing I did for my wife when she took some defense training and didn't know how many rounds each of the pistols that I gave her to use held. I cut small pieces off a roll of white electrical tape and put them on the bottom of the mags with the loaded qty written on it with a fine sharpie. You could do the same by writing 115, 124 etc on it and not having to mark every bullet you own individually.

I just use load labels for the individual box's that I load for id using MS Excel.