Seam strength is a vital factor influencing the durability and performance of garments. Among the various stitching techniques used in garment manufacturing, lock stitch and chain stitch are the most commonly employed to ensure seam integrity. Although prior studies have examined how stitch type and fabric affect seam strength, the influence of needle number has not been extensively studied. Understanding this effect is crucial for optimizing garment quality and production efficiency. This study examined how the number of needles impacts the seam strength of lock and chain stitches in 100% cotton twill gabardine fabric. The selected fabric was characterized by an ends per inch (EPI) of 126, picks per inch (PPI) of 58, a warp yarn count of 21 Ne, and a weft yarn count of 17 Ne. Fabric specimens were prepared according to the ISO 13935-2 standard. Stitching was performed using a universal sharp-point sewing needle sized 90/14 and a polyester core-spun thread with a ticket number of 40 (60 Tex). Both single and double-needle techniques were applied across lock stitch and chain stitch configurations. Seam strength was evaluated using a universal seam strength tester (Titan). Results demonstrated that double-needle stitched seams exhibited greater strength compared to single-needle seams in both warp and weft directions. Notably, the strength enhancement with double-needle stitching was not proportional to needle doubling, presenting increases of 18–27% in the warp direction and 8–11% in the weft direction. Failure analysis revealed seam rupture predominantly occurred near fabric edges stitched by the first needle, thereby constraining the reinforcing contribution of the second needle. These findings suggest that although double-needle stitching enhances seam strength, the benefits are relatively modest when balanced against increased production complexity and costs. Thus, manufacturers should consider these trade-offs when selecting stitching techniques to optimize seam performance and operational efficiency.