thousands of

thousands of是表示 数千的...（主要是形容东西，事物很多，数以千计）它已经不是确切的要表示“千”的意思了，若果是表示量词，比如一千（one thousand/a thousand，两千two thousand)就不用加s
而twenty million是表示 两千万是一个量词

也就是说，当表示量词的时候要加s,否则不用

thousands of: 数千的...

例句与用法:
1. The fair is a big gathering. Thousands of businessmen from more than150 countries and regions are here to trade with China.
本次交易会是一次盛会。来自150多个国家和地区的成千上万的商人在这里与中国进行贸易。
2. On the night of December1, 1930, a cloud of smog moved over a valley in Belgium, which caused thousands of people to become ill.
1930年12月1日晚上，一片烟雾飘过比利时的一个山谷，这使数千人病倒了。
3. The cyclone brought misery to thousands of people.
暴风给成千上万的人带来灾难。
4. Men have been making music for thousands of years.
人类创作音乐已有数千年历史。

millions of: 成百万的,许许多多的

例句与用法:
1. Millions of cattle are slaughtered every day.
每天有数百万头牛被屠杀。
2. There were millions of people in the square.
广场上有好多好多人。
3. Dinosaurs have