> れいぞうこ の なか に ケーキ は 3こ ありました.
>
> せんしゅう 、かんじ を 20こ おぼえ ました.

In the above examples, why does こ come after the number? Is it a counter? If
yes, at what instances can it be used? If not, please advise how and when it
should be used.
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numbers counters
What does it mean when a number is followed by こ
It is a counter. Ko is a kind of generic counter.For example if you were
counting paper you would use mai（枚）, cars would use（台）, animals in many cases
use hiki/piki (匹) or tou（頭）.

There are many cases, so I would recommend you look up counters. ;)

Wish you the best. 頑張ってね。 ラドより
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2014-12-13T16:35:36.707
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The こ is indeed a counter, usually written 個.

It is a _general_ counter for counting objects, much like つ. Your two example
sentences provide proof of the generality of 個, since cakes and kanji
characters have few common features.

つ is often taught as the "default" counter ("default" meaning that it can be
used in practically any situation to make yourself understood). But つ is only
used up to 10 objects. For 11 or more objects, 個 could be regarded as the
default counter.
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