how much of the Szechuan peppercorns?

Two suggestions -
You can be casual about the amount - be guided by the headnote, where the recipe author says she doesn't measure when making Chinese food (specifically referring to salt and sugar, but maybe applicable to the rest of her directions).
Start with a small amount, taste, add more if you want. (Similar recipes elsewhere use about 1 1/2 tsp for this amount of mushrooms.)