Hey devashish,
Good question - I'm more a fan of sentence/paragraph "count" than anything because it's much easier to track and much more natural, at least in my mind. I'd strive for 5-6 paragraphs of 3-4 sentences each as a goal. At maybe 15-20 words per sentence that gets you in the 400-450 range, but I think it's easier to follow at the sentence/paragraph level.
For the paragraphs, you should look for:
1 Introduction paragraph
3-4 support/counterargument paragraphs
1 Conclusion paragraph
If you've done that and taken 3-4 sentences apiece you shouldn't really need to spend any time counting words (which, if you're worried about having enough to say, will only cost you time, anyway).
I hope that helps...