I mix up a stock 10% solution (100 g/L) and add enough of that to make about 25-75 mg/L in the juice. Actual dosage is pH-dependent; you need more sulfite at higher pH. And you can vary the dosage to achieve different results, from complete "kill" to a spontaneous ferment.
My juice has a low population of wild yeast and a very high population of mold spores. Without sulfiting, mold invariably starts first and suppresses yeast, ruining the juice. So to suppress mold while retaining some wild character, I add a half-dose of the stock solution into a carboy (or tank) that is already filled with juice, and don't stir it in. This creates a high-concentration layer at the surface, without killing off everything in the juice. Typically I let that sit for a few days and then add a wine yeast (D47 is still my favorite). You can try letting it go longer and see if a spontaneous fermentation starts, but you also risk bacterial acidification. Unfortunately Acetobacter isn't particularly sulfite-sensitive.
Great discussion of sulfite use here: Sulfite - **************** wiki
Note that cider is effectively a low-alcohol dry white wine.