We require 20% extra numbers/codes in case some are discarded due to production and quality control. For example, if you order 100 cards, you will receive at least 100 finished cards, but the sequential numbering might go up to 120 because 20 of the cards had to be discarded during the production and quality control processes.
There may be certain numbers missing as mentioned above, and sequential numbers may not be packaged in order.
See: I can prepare a print-ready file myself, what do you require?, I can supply the dynamic content, can you prepare the print-ready file?