One mod, one form. Major repairs might get combined because let's say you rebuilt a stabilizer you might have repaired the stabilizer spar, replaced a skin, and replaced the hinge fitting all as part of the repair. But mods especially per an STC one per form.
I typically do 1 to 1, but in the case where more than one related STC is applied at the same time I might put them on the same form. For instance, we installed pulselites, wingtips, and recognition lights, and we opted for HID lights, the wingtips/lights were one STC but the HID option was a separate STC and the pulselite system was a third STC. I put all three of those on one form.