Several. But for an easy grapheme-phoneme correspondence, you'd better look to Spanish. You can learn that in an hour or two.
> incremental tweaks
will slowly render older texts unreadable. Nobody (ok, almost nobody) can read old Dutch texts. Even 19th century Dutch is awkward to read.
English is n:m, everything is possible and you just need to learn it.
Finnish is >99% 1:1. A native speaker can should be able to write each word correctly. In practice spelling errors do exist. Partly because in colloquial speach / dialects not all words are pronounced fully according to the rules. Partly because some people are just unable to apply the rules. Not being able to pronounce a written word seems mostly impossible.
However, foreigners should not think Finnish is easy for that reason. There are many sounds foreigners cannot pronounce correctly. And several pairs of sounds that foreigners' ears cannot distinguish.
With previous experience of English, French, German and Spanish, when I look at https://en.wikipedia.org/wiki/Finnish_phonology I don't feel intimiated like I do by https://en.wikipedia.org/wiki/Standard_Chinese_phonology or get the sense of dispair that I get from https://en.wikipedia.org/wiki/Arabic_phonology