Counters are a special form of suffix used in Japanese when counting. They often have sound changes or irregular words in the series.
For example 人(にん) nin is a counter for people. Counting down we have ...
Five people : 五人 (ごにん) gonin'
Four people : 四人 (よんにん) yonnin' (N.B. NOT shinin!)
Three people : 三人 (さんにん) sannin'
You've seen the pattern so far - what do you suppose "Two people" and "One person" would be?
Two people : 二人 (ふたり) futari'
One person : 一人;独り (ひとり) hitori'
Japanese has counters for almost everything but there are two 'generic' ways of counting.
Counting objects with つ
These use the old Japanese counting numbers (before influenced by China).
1 一つ (ひとつ) 2 二つ (ふたつ) 3 三つ (みっつ) 4 四つ (よっつ) 5 五つ (いつつ) 6 六つ (むっつ) 7 七つ (ななつ) 8 八つ (やっつ) 9 九つ (ここのつ) 10 十 (とお)
Unfortunately that's where they stop (actually there are some higher numbers but they are for specific circumstances not for general use).
リンゴをふたつください。ringo o futatsu kudasai.
Two apples, please.
Counting objects with こ
個 ko is a general counter for objects. If you can't remember what the proper counter is you will at least be understood with こ.
リンゴを二個ください。ringo o niko kudasai.
Two apples please.