I have 4 plants of my #1 female (Oso Easy Italian Ice) and tend to put only one male on each plant (Rainbow’s End on two of the plants, one on Sunrosa (Red), one mixed pot-plant red–a PorLaMar unlabeled red and I believe a Poulsen pot plant–both rich reds with HT form and good health) so no label is needed. When I try some other male, I put the initials of the male on a 1" piece of 1/4" wide masking tape using a #1 pencil and wrap it around the upper part of the stem. Each year, I try a few other females, one plant of each. I generally put my most reliable male on it (Rainbow’s End) to see how it does. Again, no labeling needed.
I graduate roughly 5% of each year’s seedlings into 5" pots (I breed minis) and for each put a large plastic stake (3"-wide 1.5 tall nameplate on each, cutting off most of the stake so the nameplate sticks up just above the soil. As I walk my graduates, which I do daily, if I notice something about it (e.g., inflorescence, disease, vigor), I note it abbreviated on that plant’s nameplate using that #1 pencil–the writing doesn’t fade, unlike garden pen.
Of course, when I harvest hips, I have a labeled baggie for each cross.