You can have as many barcodes as you can fit on a sheet of paper, as long as the barcode is big enough so that the software can read it. If is too small on the paper to begin with, when it gets scanned, the pixels are so small that they get compressed together even though to the eye it still looks like a barcode. When the scanner scans the data, it can’t read it because to the software, the barcode (depending on size) looks like a black line instead of multiple black lines. It then considers it an invalid barcode.