Ahem, apple seeds would like to argue.
Apple seeds contain about 700 milligrams of cyanide per kilo, so about 100 grams of apple seeds should be enough to dispatch a 70-kg adult human, but that’s an awful lot of apple cores even if you don’t eat the rest of the apple first. In addition, the seeds would have to be pretty finely crushed to let the enzymes get to the amygdalin at all. All in all, you’re safe eating the occasional apple core. I’ve done it for years. Just don’t try eating a bowl of freshly crushed apple pips.
If a seed weighs 0.7 grams, then you’d need to munch your way through 143 seeds. Apples can contain anywhere between 2 and 20 pips, but a typical supermarket apple will contain about 8. So you’d have to eat about 18 apple cores in one sitting!
The cyanide I used, sodium IIRC, came in a bright white package with a great big skull and crossbones on it.
No apple seeds in it at all.
L