The characters are mostly 1-1. There are a few exceptions, but usually one is a lot more typical than the other, so reading it with the typical reading won't usually get you in trouble
While not as egregious as Japanese where characters can have 15+ readings, the number of exceptions certainly are not few. Below is a link to the official table of words with multiple pronunciations in standard Mandarin.
While I couldn't find any characters with more than three or four readings in this list, the Taiwan list (https://language.moe.gov.tw/files/people_files/%e5%88%9d%e7%...) has one character with five readings (著) and one with six (和). Still a long way from 15, though.
It's worth noting that the aforementioned list is the unified pronunciation list that was published in 1985 by the Ministry of Education. The reason why you see some words only having a single (unified) reading in that list is due to the necessity of having to unify them in the first place, although there are still quite a few words with multiple readings. Keep in mind that there was no official language of China until 1932. Without going into detail about how pronunciations evolved with the change of dynasties and how China actually has 300+ spoken languages, the need for a unified pronunciation stems from the fact that many people in China, historically and even today, do not speak standard Mandarin as their first language. In other words, prior to 1985 it was much more chaotic. If you want a more up-to-date comprehensive list of words with multiple readings (多音字) you can find it below (although this is not an official government list). I've linked directly to the common words of which there are 106 (although the page does not define what is considered "common").
The multi-sound characters are common in usage. For example 长 can be cháng meaning "long" or zhǎng meaning "to grow". 行 is xíng "to walk" or háng with no real single coherent meaning, appearing in compounds like 银行 yínháng "bank" and 行业 hángyè "profession". All of these are very common usages. In context they are essentially never ambiguous, but if you are going through character-by-character it's not going to make sense.
