1(lit.) by the thousands and tens of thousands (idiom)
2(fig.) untold numbers; innumerable; countless
Examples
chéng qiān shàng wàn成千上万de的shì bīng士兵hé和shì mín市民sǐ死qù去le了。
成千上万的士兵和市民死去了。
Thousands upon thousands of soldiers and civilians died.
每年有成千上万的外国人访问日本。
Thousands of foreigners visit Japan every year.
成千上万的人对这个广告很失望。
Thousands of people were very disappointed with this advertisement.