You need one barcode per SKU (product variant) so the retailers can identify different sizes and colours.
The good thing is barcodes are cheaper when you buy in bulk. Also barcodes never expire so you can buy a set of say 100 barcodes and use 50 now and keep the other 50 to use as and when you have new products.
In general, the easier you make it for your retailers, the more happy they are likely to be to stock and keep stocking your product. So, if at all possible we would recommend having different barcodes for each variation.