I want to run simple website (simple startup introduction page + admin page) using ubuntu 14.04, nginx, django and mysql on 5$/month plan has 512MB RAM. If the number of visitors per day is 1000, this plan is ok?.
If this was me personally, I would choose to be on the safe-side and use the next tier up purely because you'll only get a small amount of allocated resources. If you're planning on heavily utilising the running database or could experience large traffic bursts it'll just act as a safe guard.
However, if your budget is low and you're certain your application isn't resource intensive the 512mb droplet should be capable of handling that amount of traffic with a breeze. 1000 basic page requests to a server is nothing, but problems can arise when you start using high amounts of resources to serve them.
Good luck with your project :)