I am not actually sure on the number to use, but I would say, use it as many times as you want as long it flows well...but I wouldn't use it 50 times in one post lol. So, I guess what I'm saying is 1 time every hundred words sounds good, but I usually only end up using my low hanging long-tail keyword about 2-3 times per post. I would definitely use the keyword in the title and in the first paragraph for sure.

Get keywords to naturally merge with the content. So the keyword has to make human sense. Also, you can place keywords in the titles, alt tags, descriptions when you add images. Don't spam it. Also, having the keyword in your meta descriptions help.