I have some berries of alpine juniper. I read that in big quantities they can be toxic. So for the moment I tried them in small amounts. Last time I took a couple of pinches, about ten berries, I crushed them I threw away the peels, mixed the rest with the food and cooked everything. I liked the taste, but it was faint, not very spicy.
Roughly how much juniper could I add to the food and still be on the safe side?
All juniper berries contain thujone, which is the primary source of toxicity concern. Thujone is psycoactive in minute quantities, but in larger ones is a convulsant and disruptor of the digestive system. Different varieties of juniper have different concentrations of thujone and other chemicals (like sabinylacetate), resulting in vastly different levels of toxic risk (particularly, Savin Juniper is dangerous with just a few berries).
Alpine juniper are Juniperus communis, which is the "safe" variety usually used for culinary purposes. So you're OK there.
Per the excellent scientific papers linked by @Ecnerwal, toxicity levels of Juniperus communis fruit (Juniperi fructus) are extremely low. To quote the veterinary paper:
the Committee for Veterinary Medicinal Products concludes that there is no need to establish an MRL [maximum residue limit] for Juniperi fructus
... in other words, toxicity is low enough that it's not worth establishing specific safe amounts. For example, strong alimentary reactions in rats were observed only at a level of 0.5% juniper oil in food -- the equivalent of consuming a teaspoon of pure juniper essential oil in your meal. Or, more to your question, two US cups of juniper berries. Abortifacent reactions with Juniperus communis were even higher.
Similarly, US food authorities regard Juniperi fructus as Generally Regarded as Safe, and do not establish specific amount limits.
So the overall question of "how many juniper berries is potentially toxic" is "more than you're going to be willing to eat".*
All that aside, the flavor of Juniper berries should be very strong. I usually only add about 4-7 of them to a large pot of rotkhol. As such, if you're getting little flavor from 10 berries, it may be that you don't want to use these particular berries because they're just too bland.
(* caveat: you, personally, might have a specific allergy to juniper, which would lower the amounts that would provoke a reaction)
