I don't measure my charcoal real accurately. Plus, many times there is leftover charcoal from the previous bake that doesn't provide as much heat.
But, as a guesstimate, I'd say I put in 2-3 qts of charcoal to start with. That will be enough for the 45-60 minute preheat. Then, 5-10 minutes before cooking, I'll add in another couple double-handfuls. That usually gets me through 5 pizzas and I'll add charcoal every 5 pizzas.