Answer quickly: How many vowels are there in English? If you said five, you’ve misunderstood something about writing.
First there was speech, and then there was writing. While speech developed hundreds of thousands of years ago, the first writing appeared only 5000–6000 years ago. In literate societies, including the Anglosphere and the Sinosphere, writing has had a profound influence on the use and perception of language. When we learned English, we were taught that five (or seven) letters are used to represent vowels. Actually, if your experiences were like mine, you were told that the letters a, e, i, o, and u, were vowels. But of course, vowels aren’t letters, but sounds. By the way, there are about ten vowels in General American English.
Similarly, when we learn Chinese or Japanese, we may encounter homographs, characters that correspond with more than one pronunciation. You may have been taught that these characters inherently exhibit them, often to correspond with multiple meanings. Such an interpretation is backwards.
Here’s what really happened. In most cases in Chinese, there was once a word with multiple ways of inflecting it. An example in English is rain. You can use it as a verb, “Looks like it will rain today,” or a noun “I’ll be walking in the rain.” In both cases it sounds and is written the same. Different inflections might be rains, rained, and raining, to be used in other ways. Old Chinese (the language on which the Chinese writing system is based) may have had a system of inflections. There was a word that sounded something like *gwjaʔ and meant “rain,” the noun. (The asterisk means that this is a reconstruction and isn’t attested.) This could be inflected by adding *-s to become something like *gwjaʔs, which meant “rain,” the verb. In this way, where 雨天 “rainy weather” may have been *gwjaʔ tʰin, 天雨 “it [heaven] rains” may have been *tʰin gwjaʔs.
As it turns out, such affixes evolved regularly to correspond with certain modern tones. In the case of Mandarin, the Old Chinese *-s suffix usually corresponds with the Mandarin 4th tone, and so, there are two words in Mandarin written 雨: the noun yǔ, corresponding to Old Chinese *gwjaʔ, and the verb yù, corresponding to Old Chinese *gwjaʔs. The most common homographs in Mandarin are the result of different Old Chinese inflections. However, such cases are rare. Often, different inflections will obtain new ways of writing them. For example, *kens “see” and *gens “appear” were both written 見. These correspond to Mandarin jiàn and xiàn respectively. 現 appeared later to write *gens (Mandarin xiàn), while 見 was used for *kens only. And of course, different inflections in Old Chinese are not always preserved in Mandarin. This sort of information will be in the Expert Edition of our dictionary.
The topic of “readings” is more common when studying Japanese. As you may know, Chinese characters were imported to Japan, first to write Chinese, then to write Japanese. The result was that modern Japanese uses Chinese characters to write both Chinese loanwords and native Japanese words, in the same way you could use 雨 to write the English word “rain.” In the case of 雨, there was already a Japanese word meaning “rain” /ame/, and there were the newly imported Middle Chinese words meaning “rain” which sounded something like /ɦju˩˥/ and /ɦju˥˩/ (Mandarin yǔ and yù respectively). These Chinese words were both approximated into Japanese as /u/, and now the character 雨 is considered to have at least the two “readings” /ame/ and /u/.
In general, native Japanese words written with Chinese characters are said to be “meaning readings (訓読み kun’yomi),” while Japanese approximations of Chinese words are “sound readings (音読み on’yomi).” As you may suspect, there may be multiple “readings” of both types. Multiple on’yomi may arise from Chinese homographs or different periods of borrowing, while multiple kun’yomi arise naturally from using Chinese characters to write Japanese words. Consider, for instance, if English used Chinese characters, and whatever roughly meaning “ball” was written 球 (Mandarin qiú), and let’s import the Chinese word qiú on top of that. We’d have the on’yomi qiú, and various kun’yomi presently written “ball,” “glob-,” “spher-” etc.
Remember, though, that these are not really “readings” of characters, but different words that happen to be written using the same characters.