I have written so many posts, I can roughly tell if my post is 50, 100, 200 or 300 words just by the feel of it. Previously, I use a WordPress plugin that shows the wordcount after I published the post. I need to add some codes on the single.php and the number of words will show underneath the post.
However, I found an easier plugin that actually counts the number of words as we write. This is better because we can tell right away if we have crapped too much. Which is the case that happens to me all the time. If advertisers asked for 50 words, I will give 100. If they want 100, I normally ended up with 200 words.
Remember not to overshot your paid posts too much because a lengthy post may drive your readers away. So try to stick close to the number of words required. But in my personal opinion, I can never get my message across with only a 50 words post.
If you wish to find a cool tool that helps you to count as you write, check out Leprakhauns Word Count, a WordPress plugin. Instructions on his site.