It does not really matter, because when you are starting from the ground zero, you will not have many visitors (if any). The number of posts depends if you add them manually or automatically, but if you change video titles and other stuff to maximize the search engine optimization, five to ten videos should be enough.

You should write them naturally, not just put keywords in every sentence, if you do it that way, then you better do not have them at all because Google will notice that and will slap you with a keyword stuffing penalty. The description does not have to be long, mine are about twenty to thirty words long but they are always unique. You might visit MyLust.com for some samples. Long tail keywords are proven to be a good way to get organic traffic and you should definitely go after them first because they are more reachable by an average affiliate.