No. Kana Kanji
1 ひとつ 一つ
2 ふたつ 二つ
3 みっつ 三つ
4 よっつ 四つ
5 いつつ 五つ
6 むっつ 六つ
7 ななつ 七つ
8 やっつ 八つ
9 ここのつ 九つ
10 とお
How many いくつ 幾つ  

This is one of the things you just need to memorize. This counter is used just to count objects from 1 to 10.

When counting beyond 10 things, つ counter is no longer used - instead counter is used (じゅう一個いっこ, じゅう…)


Check out other counters.