First, remember the basic rule of software development - you can have it cheap, fast, or good, pick 2. I've been a subcontractor on a number of projects over the years, and doing it solo made it cheap and, I believe, good (evidence = repeat customers), but the projects took a while. So, the number of coders you need depends on how fast you want the project complete. Make sure your coders are competent IN THE SPECIFIC AREA of your project. They might be great at developing games but have no idea about a DNN. Don't go too cheap - results will be unreliable. I have done hourly and by-the-job, I like either way and am honest in my time, but often until you build a relationship with the coder you may prefer job pricing. As a coder I build some extra time in on my estimates for job prices, to give myself a buffer.
Hope this helps.