I am not a proponent of top dressing to cover the seed to substitute for proper soil preparation. I think you would need too much. If you planned to bury the seed a quarter inch deep. My estimate is 21 cubic feet per 1000 sqft. (That would be three-quarters of a cubic yard per 1000 sqft. (Around 1650 pounds).
http://www.ask.com/questions-about/W...c-Yard-Topsoil
And it is nearly impossible to spread it evenly.