How many peppercorns to a 2 litre Beef Casserole?
There's no real right or wrong answer here, it's totally subjective as I have no idea what you'd consider "too peppery". I like pepper so too peppery for me is a lot, whereas some of my family can't tolerate any heat, so too peppery is basically the merest hint. I'll try and scale it for you depending on how much heat tolerance you (or your guests) will have:


Likes it hot, real hot. Eats spicy food often and adds hot sauce to everything: don't just add peppercorns, add chili peppers as well. Probably not you
Likes a bit of heat occasionally, gets spicy dishes when going out to a mexican or thai restaurant but doesn't subscribe to hot sauces weekly: 7-8 whole peppercorns plus one whole chili. You pick the chili out whole, and try to keep it from bursting. Adding a whole chili gives a bit of chili flavor but without the heat
Occasionally will go out on a limb and have medium salsa on a chip, but that's where it ends: 5 peppercorns
Mild salsa only, picks the jalapenos off nachos and gives them away: 2 peppercorns
Won't touch anything that even has a chili in the same proximity: 0 peppercorns


I like pepper and spicy food, but I don't like dishes with too much pepper flavor, it can be way too strong. I stop at maybe 7-8 peppercorns and if I want more heat add chili.