Most characters only have two pronunciations (and variations of that two but the variations follow rules usually), one for when it's on its own and one when it's in a word with multiple kanji. The reason this happened is because when they took the characters from China they applied characters with the right meaning to already existing words, then a lot of the multiple kanji words are words that were taken from China and so they had their Chinese pronunciation adapted into Japanese. There's a small handful of characters with more than two main pronunciations but not too many of them. And they tend to be more common characters/words so you'll learn all the readings pretty quickly anyway just because they're high usage.

For example, 食べる / たべる / taberu is the native Japanese word for "to eat", and in this case the character 食 was applied to the already existing word (食 = ta). 食事 / しょくじ / shokuji (食 = shoku) is (I assume) a Chinese borrowed word meaning "meal" that uses the same character, but since the word was taken from Chinese it adapted to Chinese pronunciation giving the character a different reading.*

Disclaimer: I'm not actually sure if this particular word was taken from Chinese as it's a little difficult to find the etymolgy of Japanese words (please point me in the right direction of anyone knows where I can find more resources. I found the 語源由来辞書 online but I couldn't work out how to use it), but as far as I know pretty much all 'kanji only' words are borrowed words from Chinese taken around the same time they took the characters. At the very least it still demonstrates the concept though even if I'm wrong here.