How many top plates can I add

RAISE THE CEILING?

You may raise the ceiling by having as many as 4 top plates but you may create a structurally weak foundation for your second story by doing so. By stacking the plates, as you suggest, you are essentially stacking blocks beneath your second story. Under the right conditions, such as a high wind or earth tremor, those blocks may allow the upper story to shift or shear under such pressures and damage your home. My suggestion would be to use a vertical 2x12 band, properly attached to your existing top plate, with  2x8 floor joists raised to the top of that band as the basic floor framing for the second story. Following this method of construction will create the structural stability you need for the second story and raise your first floor ceiling by approximately 3-3/4 inches. If you require an even higher first floor ceiling, you may use laminated beams, (available in various heights), in place of the 2x12 band mentioned previously. The height of the floor joists for the second story may be increased to 2x10  or even 2x12 as well, depending on the height of the laminated beam and the desired height of the first floor ceiling. In either case, I suggest that you use the appropriate size of joist hangers for your second story floor joists. The latter method is the one I would use if I were doing this addition on my own home. 

Best wishes with your project,

Prodigal Son