My instinct tells me "several thousand (of them)" is correct, same as I would say "several hundred (of them)" or "several million (of them)". However I cannot explain why :(

I agree with Shizenai and TeacherJan. It is "several thousand of them," without the S. The reason is that hundred, thousand, million, billion, trillion, etc., only take an "s" when used alone. That is, we say there are hundreds, thousands, millions, billions, and trillions of stars in the galaxy. However, when these numbers are preceded by an adjective that denotes quantity, we drop the s. For example, we say: Three thousand, five million, six billion, seven trillion, etc, but *not* three thousands, five millions, six billions, and seven trillions. (Except perhaps when we want to count the number of these words in a text, say, for example, there are three "hundreds" in this paragraph). Likewise, we say several hundred of them, several thousand of them, etc. I will say, however, that saying "several thousands of them" probably does not sound terribly wrong to many people.

Google is a very useful tool for questions of usage: "several thousand" - about 2,810,000 results "several thousands" -about 427,000 results You can examine (1) the numbers (as above) and (2) the contexts and you will learn a great deal. If you'd like to see a more detailed discussion, go to http://forum.wordreference.com/showthread.php?t=952435 .

Yes, because "several thousand" is being used as an adjective, the plural -s is dropped (just like "several hundred people vs. several hundreds).

I see, shizenai. Linguistically speaking, what is used by the majority *is* correct. However, what I wanted to show here was that internet search engines are a very useful tool with questions like this one. It's one way native-speaking writers, editors and proofreaders check for current usage of words, punctuation and more. By running a search, you can see the way words are used in context and (in this example), whether adding "s" creates a difference in meaning or not.