The answer depends on your TA level, but with a TA of 100 (and CYA of 40) it would take 51 fluid ounces of Muriatic Acid (31.45% Hydrochloric Acid) in 14,000 gallons to lower the pH to 7.0. If the TA were 150 ppm, it would take 73 fluid ounces. If the TA were 70 ppm, it would take 38 fluid ounces. If your CYA level is higher, say 80 ppm, then starting at a TA of 100 would take 60 fluid ounces to get from a pH of 7.6 to 7.0.
When I said it takes the same amount of acid to lower the TA, that means the total amount for the entire process, not the amount needed to get to a specific lower pH for the first step.
The total amount of acid that will be needed during the entire aeration and acid addition process to lower the TA from 150 to 80 is 250 fluid ounces (in 14,000 gallons). To lower from 120 to 80 it takes 143 fluid ounces. To lower from 100 to 80 it takes 72 fluid ounces. That should give you a rough idea of how much acid will be needed depending on your situation (you didn't say what your current TA level is).
When adding acid, especially when you haven't done this a lot, it is best to add half the calculated amount (for each step) and remeasure pH after at least 30 minutes or an hour (with the pool pump running and good circulation) and see where you are at.
Richard