As a noun, 'thousand' (as well as 'hundred,' 'million,' etc.) can be made plural, followed by 'of,' and modified by quantifiers such as 'several,' 'a few,' and 'many.'
当thousand(也包括hundred,million等）用作名词时，有单复数变化，后面跟of短语，thousand前面常用'several,' 'a few,' and 'many.'进行修饰。
例如：There are several thousands of people protesting on the street.（有好几千人在街上示威游行。）
...,but tens of thousands came.(结果来了成千上万的选手。）
As an adjective, we use 'thousand' in the singular, and would say, 'several thousand people...'
当thousand用作形容词(即后面直接跟它所修饰的名词）时，它就用单数，比如说：several thousand people（好几千人）.
因此下面的用法都是正确的：
- a thousand(hundred...) of sth/sb
- a thousand(hundred...) sth/sb
- several thousands(hundreds...) of sb/sth
- several thousand(hundred...) sth/sb