1 tire line. 235/50R21 tires sized to your wheels and your car.

A 235/50R21 is 235 mm wide with a 118 mm sidewall (50 series) on a 21 inch wheel, 30.3 inches tall overall. A tire size only makes sense next to a wheel width and a car. 235/50R21 mounts on a range of widths, and the overall height decides whether the speedometer and the fender stay happy.
1 line carries 235/50R21, from Michelin. By type that is 1 performance summer. The tire has to stay near your car's stock height and mount on your wheel width, so pick the car and the wheel set and the page keeps only the 235/50R21 lines that do both.