Does Heirloom add extra seeds for germination losses?

Yes, with a safety margin you set once per crop, so you sow enough for your losses without doing the math.

The Seeds safety margin is a percentage. It starts from the Nursery chart of your Charts, where you set it per crop, and you can change it on each planting, in its nursery tab. Heirloom counts those extra seeds in your seeds, your trays and your nursery space.

More in Forecast your nursery space and trays and How Charts work.