How much potassium metabisulfite/Potassium Sorbate to stop fermentation?

Short answer: none. It is a common misconception that metabisulphite stops fermentation. It doesn't. It is an oxygen scavenger and a bacterial inhibitor.

As already mentioned above you should use potassium sorbate to halt fermentation.

One tablet in one gallon of must yields 150 ppm total sulfur dioxide.

Those instructions are on the bottle of Campden tablets, which are potassium metabisulfite.

Here's a source for Campden tablets.

It is common to use Campden tablets in wine, mead, and sometimes cider. It is less typical to use in beer, although it can be.

Potassium Sorbate also has instructions right on the bottle.

"Potassium sorbate, aka "stabilizer," prevents renewed fermentation in wine that is to be bottled and/or sweetened. Use 1/2 teaspoon per gallon."

Here is a source for Potassium sorbate.

It is my understanding that you use one or the other, either Campden or Potassium sorbate, not both.

Potassium sorbate and potassium metabisulfite accomplish different things, and can be used together to both stop fermentation and prevent renewed fermentation.

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Hope that helps.

I am assuming that you are making wine. For beer, neither sorbate nor SO2 have a place, since homebrew either creates carbonation from fermentation in the bottle or via CO2 injection in a keg, and usually there is no residual sugar to worry about for "re-fermentation" later in either case given timelines for deciding when to bottle/keg, meaning re-fermentation can be avoided without chemicals.

For wine, honestly, don't be in a hurry. The only thing to stop fermentation here is 1) there is no more sugar in the must for the yeast to "eat", or 2) you filter the yeast out (if you really are in a hurry --> but costs $$ for the pump and filters).

Sorbate will not stop fermentation forever, only for a "while" -- but may be enough until you drink everything, in which case the Web is full of doses you can attempt. But SO2 is intended mainly for controlling bacteria and is highly unreliable for "preventing yeast from multiplying" if you don't do residual sulfur tests.

Plus, "preventing yeast from multiplying" is not the same as preventing fermentation.

So neither sorbate nor SO2 are fool-proof against re-fermentation if there is a significant residual sugar left.

Either use a yeast that dies at your top-level alcohol requirement (Lalvin D47 and 71B stop at 14%), or use a good filter system, or build a mini-test-lab in the basement.