Carpet is available in 12’, 13’ and 15’ widths. Unless the room to be carpeted is narrower than these widths, the carpet will be seamed. With looped or low-profile patterned carpets, you may have visible seams. The degree of their visibility depends on texture, color, lighting, and furniture placement.
Be aware, as carpet bends over stairs, its backing may show depending on texture, color, and density. You should also know that looped carpet can snag, particularly at a seam or at a carpet transition.