Should I use On reading or Kun reading for numbers?


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?




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`, ...

