First, in case you don’t already know what cyanide is, let me tell you. Cyanide is a strongly poisonous substance. Just a few grams of it are easily enough to kill a human. Read more about the harmful effects of cyanide on the human body here. In fact, cyanide is so poisonous that it has been a favourite of mystery novel writers for decades. Uncountable characters have been killed off using this poison, be it dissolved in a glass of wine, or administered directly. Cyanide is also what Adolf Hitler, Alan Turing (supposedly) and several others used to kill themselves.
Now that you know what cyanide is and what it is capable of, you must be thanking your stars that you are still alive having swallowed so many apple seeds in your lifetime. However the truth is that the amount of cyanide present in apple seeds is very small. You would need around 100 grams of apple seeds to kill a single human. Further, even if you did swallow these many seeds, you would probably still not die, because the hard outer coating of the seeds would protect you from the cyanide inside. In conclusion, unless you grind 100 grams of apple seeds into powder and consume it all at once, you should be fine.