The 88 Spider owners manual lists the tank capacity as 12.2 gals. including a 1.5-1.8 gal reserve. However, a steady on low fuel level light does not necessarily mean that there's a full 1.5-1.8 gal reserve as not all low fuel light switches have the same setpoint.
The amount of fuel still left in the tank when light goes steady on, as well as miles to empty, can be calculated by knowing the tank capacity (12.2 gals.) and your MPG.
Series 1 Euro 1750 GTV
Series 2 USA 1750 GTV
Series 3 Spider Veloce