A numbered parallel is a version of a card produced in a stated quantity, often printed directly on the card as something like 25/99 or 1/10. It may differ by color, foil, pattern, photo treatment, autograph, memorabilia, or set insert structure. Numbering can matter, but it does not make every card valuable. Use sports card shops to inspect examples in person and ask how the shop distinguishes a true parallel from a base card or common insert.
Value depends on the player, year, product, condition, grade, sport, design, and actual collector demand. A low-numbered card of an unwanted player may sell for less than an unnumbered rookie of a star. The numbered parallels guide and sports rookie cards guide explain how to compare exact cards. When buying or selling locally, bring recent sold comps for the same player, product, and parallel rather than treating all serial numbers as one market. Ask to see the serial number before pricing or trading for any claimed parallel.