Measure words: number + measure + noun

The rule Number + MEASURE + noun: 三个人。 Never 三人。
个 is the general measure. 本 for books, 杯 for drinks, 口 for family members, 点 for o'clock.
When counting, two is 两, not 二: 两个人, never 二个人。
这 and 那 need a measure too: 这本书, 那个人。