How egregious a mistake is it to use the wrong reading for a number when counting?


I've learned that the readings for numbers in the context of counting have
many irregularities to them (like 二十日 for example).

I'm wondering how noticeable it would be to get them wrong. For instance, if
someone learning English were to say "He run outside!" that would be a mid-
level mistake-- the sentence still makes perfect sense. If someone were to say
"How me go up?" that'd be pretty broken. We can make sense of it but it's a
heavily misworded sentence; they sound like they only know a few words of the
language. And then something like "Me and Jon love chocolate" is an almost
irrelevant grammatical mistake that many native speakers make constantly
without anyone noticing.

So, I'm curious, with a sort of scale like that, to a native speaker's ears,
how bad is it to get the pronunciations wrong of the many irregularities
explained on [this
page](http://www.guidetojapanese.org/learn/grammar/numbers)?

E.g. ふつにち instead of ふつか, or にじゅうか instead of はつか....there are so many
possible examples of ways to mistake pronunciations of dates, times, and
counting.




Reading 二十日 as にじゅうにち may be relatively tolerable; I think elementary school
kids make this mistake all the time. When I hear an adult say this in casual
chats, I would notice it, but probably ignore it. にじゅうにち is "regular", so
everyone can understand what it means.

Reading 二日 as ににち, 三日 as さんにち, and so on are less tolerable IMO. Teenagers
usually don't make such mistakes. But it always makes sense anyway because
it's "regular".

にじゅうか and ふつにち (and さんか, よんか, いつにち, ...) are just plain wrong. The listener
wouldn't probably understand what you're trying to say.




Reading 二十日 as にじゅうにち may be relatively tolerable; I think elementary school
kids make this mistake all the time. When I hear an adult say this in casual
chats, I would notice it, but probably ignore it. にじゅうにち is "regular", so
everyone can understand what it means.

Reading 二日 as ににち, 三日 as さんにち, and so on are less tolerable IMO. Teenagers
usually don't make such mistakes. But it always makes sense anyway because
it's "regular".

にじゅうか and ふつにち (and さんか, よんか, いつにち, ...) are just plain wrong. The listener
wouldn't probably understand what you're trying to say.

