There's a tradeoff. Fewer posts per page means more pages, and some say that that has a positive impact on page rank. Shorter pages also load more quickly, and that might reduce the blog's bounce rate. More posts per page allows for greater keyword repetition and better on-page optimization. It also creates a long sidebar with plenty of room for lists and widget. I opted for a long page, and I'm happy with the way it's working out.
It depends on the average length of each post. If the post is around 10-15 lines then 10 post is kind of OK or else 5 should be the best choice.