There should be a spray chart that comes with your sprayer that shows the gallons per 100 SQ/Ft, gallons per 1000 SQ/FT and gallons per acre, per nozzle based on pressure and speed. It should also be on the sprayer/nozzle manufacturers website if they have one. This is also dependent on the nozzle height, and the correct height should be listed with the chart. I spray at the lowest pressure I can to avoid the ultra fine mist and too much drifted overspray. For 2,4-D you just want it to kind of rain down on the broad leaves anyway which will naturally catch more of the falling spray anyway. As an example, on my chart, if I spray at 20PSI and travel at 3 MPH, I will deliver 20 gallons per acre per nozzle, which is a good quantity. It dosn't matter how many nozzles, more just means you will cover the acre quicker at a given speed.
I mix 32 OZ(2 pints) of 2,4-D Amine to 20 gallons of water(8 OZ/5 gal). I have a 25 gallon sprayer so I add 40 OZ to that 25 gallons. IF I travel at 3MPH, 20 gallons, and the 2 pints of 2,4-D it carries will be distributed over an acre. Sprayed on a warm sunny day, this minimum ammount seems to work real well for my situation.
The key, at least for me is to maintain that speed as best I can. I tow my sprayer behind my garden tractor which has a hydrostatic drive unit. because of this, it is not directly coupled to the engine, so the tach is not real good for maintaining speed, and the tractor will speed up going down hill. To help maintain a steady speed when spraying, I added a bycycle speedometer(about $9 at wallmart) which counts the passings of a magnet that I attached to one of the rear wheels. With the wheel diameter loaded into the speedo, it converts the rotation count into MPH.
That 8OZ/5Gal mix is also good for spot spraying, which is what I do close around the house, and how I avoid killing my wifes flowers

Just wet the weeds with a little bit of spray(it goes a long way) It also greatly reduces the ammount I put down around the house, as the dog and cats use this area regularly.