Should I use On reading or Kun reading for numbers?

Question

I've just started to learn Japanese, and I've been looking at the jōyō kanji. Most of the words have on reading and kun reading. I was wondering, which one is most popular and when should I use one over the other?

Answer 1

As a general rule... 

• You will use ON readings for numbers most of the time when using specific counters. As @summea mentioned, there are numerous times when certain numbers have alternate pronunciations.
• You will use the KUN readings when specifically using the "generic" counter (一つ, 二つ, 三つ, ...).
• Then there is a third group of counters that use KUN for the first two, and then switch to ON from there on. I'll have to look up some other examples in my reference book later, but the one I immediately think of is when counting "groups" (グループ).
  • ひとグループ, ふたグループ, さんグループ, よんグループ, ...

Answer 2

There will be times where you'll want to use both ON and KUN readings for numerals in different situations.

For example, when you are counting objects, you might be using KUN readings, like this:

一つ{ひとつ}、二つ{ふたつ}、三つ{みっつ}

If you were looking at months of the year (January, February, March, etc.) you would most likely be using ON readings, like this:

一月{いちがつ}、二月{にがつ}、三月{さんがつ}

And do keep in mind that there are a few "weird" numerals (4, 7, and 9) that have their own sets of rules. Sometimes the 4 will be read as し and other times 4 will be read as よん. Sometimes 7 will be read as　しち and other times 7 will be read as なな. Sometimes 9 will be read as きゅう other times 9 will be read as く and even other times, 9 will be read even differently (for example, ここのつ for "9 things"; and this can happen for even other numerals! For example: 1, 8, 10, etc.)

As you continue to study, it is helpful to learn both the ON and KUN readings of numerals (even though it is more content to learn) because you will use both, in everyday Japanese.

More information about Japanese numerals can be found on this Wikipedia page, as well.