One row per jersey. Use Tab to move across fields.
Your file needs one line per jersey, with these four columns:
size — XS, S, M, L, XL, 2XL, 3XL (XXL / XXXL also accepted)
name — up to 24 characters, leave empty for no name
number — digits only, up to 3, leave empty for no number
quantity — whole number from 1 to 99
Commas, semicolons and tabs all work as separators, so a file exported from Excel or Google Sheets imports as-is. Lines starting with # are ignored. Download the template for a ready-made example.