Fuel octane has been a fairly misunderstood thing over the years, especially among neophyte enthusiasts. Octane is the fuel's resistance to detonation under stress; it's something you can't do without when you need it, but nothing that will do you any good if you don't. There's more than one way to bump your fuel's performance, and not all are created equally.
The single biggest variable here is the quality of the octane booster. The cheapest boosters are really just ethanol and methanol, which work to boost octane to some degree, but ultimately decrease the fuel's top-end power potential because they carry fewer BTUs of energy per unit volume. High-quality octane boosters, though, will give you a safer octane boost by using less alcohol and more specialized additives. So, quantities will vary by the manufacturer.
Torco Racing fuels Accelerator and Kemco Octane Supreme will both require about 3 ounces of additive per gallon of gas to increase octane from 93 to 100; Torco's maximum gain from 93 octane is 14 points, but only if you add about 6 ounces of additive per gallon. At the end of the day, though, octane booster should be considered a second choice to just buying 100-octane fuel in the first place. The 93-otance pump gas you start out with is chemically compromised to meet a price point and government emissions regulations. Dedicated race gas generally isn't. You can bump 93 octane up to 100 octane with a good booster, but that isn't going to increase the overall quality of the fuel.