Just use“Ko”when you don’t know how to count correctly
Counting in Japanese often confuses those who are learning the Japanese language. When you count dogs in English, you just add the noun after the numbers, eg: “one dog,” “two dogs,” “three dogs,” etc. However, in Japanese, the counting method differs depending on the objects being counted.
Taking a dog (inu) as an example, we don’t say “1 inu,” “2 inu” or “3 inu.” Instead we use the term “hiki” after the number, ie: “1 hiki (pronounced ppiki),” “2 hiki,” “3 hiki (pronounced biki).” Usually “hiki” is used for animals, however, it is common to use “tou” for large animals such as horses and cows, and “wa” for birds.
There are many different terms used for counting including “ki” for airplanes, “dai” for cars, “ken” for houses and “mai” for paper. Although it is helpful to memorize these words one by one, it is safer to use “ko” after the number when you are not sure which term to use. There is one more problematic aspect of the Japanese language — the pronunciation. The pronunciation of “hiki” differs depending on the numbers, for example: “i-ppiki,” “ni-hiki,” “san-biki,” etc.
You do not have to pay much attention to this, since the ‘counters’ are changed only for the purpose of making pronunciation easier. As you can see, it is easier to say “i-ppiki” instead of “ichi-hiki,” “san-biki” instead of “san-hiki,” and so on. There are plenty of other similar examples. When counting bottles with the term “hon,” the counter changes as follows: “i-ppon,” “ni-hon,” “san-bon.”