Is there any plausible way of expressing a sentence like
There are thousands of people in this city
Could you express that with numbers? Using the figure 1000 would make it look like "a thousand people in this city"
As an answer to your question ,the definition of thousands based off of dictionary.com
the numbers between 1000 and 999,999, as in referring to an amount of something: Property damage was in the thousands.
The plural number form to express thousands:
1000s American and common universal according to spellcheckers 1000's British (old school) Although using an apostrophe-S to make a number plural is somewhat common, it is incorrect.
Just one quick bit of advice: If you state "1000s", make sure you have at least 2 thousand, the minimum required for plural of one thousand to make it grammatically correct.
Thus: 1000s is the grammatically correct answer.