Enter how many EAN-13 barcodes you need. Each one is random and unique, laid out one per label for your thermal printer.
In the print dialog set margins to None, scale 100%, headers/footers off. Thermal roll layout: paper size = your label size. A4 layout: paper size = A4, and the page fits as many labels as the size allows. Test with one label first and scan it before printing a big batch.
Each code is built from a scrambled minute-by-minute time stamp plus secure random digits, so a code made in one minute can never match a code made in any other minute (for about 19 years), even on another device or after clearing browser data. Within the same minute, this browser also remembers every code so none repeats. EAN-13 codes start with 2 (the range reserved for in-store use), so they won't clash with real product barcodes.