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

Question

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?

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.

Answer 1

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.