个【gè】

个 is the measure word for people and for nouns with no measure word of their own. It goes between a number and the noun.

In speech it is usually toneless, ge rather than gè.

A number cannot touch a noun directly

An ordinary noun needs a measure word between the number and the noun, and 个 is the one to reach for when nothing more specific applies.

• × 三学生 → 三个学生

Before a measure word, "two" is 两: 两个, never 二个.

It is not a universal measure word

个 is the default, not a wildcard. Nouns that have their own measure word keep it, and 个 in its place sounds wrong.

• × 一个书 → 一本书
• × 一个纸 → 一张纸
• × 一个衣服 → 一件衣服
• × 一个笔 → 一支笔

个 with 这, 那 and 哪

Demonstratives need a measure word too, and 个 is again the default.