How do I count Ram and CPU to my site? Accoriding to number of unique visitors per day and sustained visitors ?
Well, if you consider 'sustained visitors' to be concurrent visitors, you can work that way?
I would suggest allowing 512Mb or so per concurrent visitor and either configure PHP-FPM with the static process manager based on the RAM you have available or, if you're using mod_php5, setting the MaxClients in Apache equal to the amount of 512Mb-Magento processes that you can support. This way you won't overcommit yourself and you can make reasoned decisions about how much RAM to have based on the average number of concurrent visitors your site is getting.
if you go the mod_php5 / MaxClients route you will probably want to serve assets via another webserver (like nginx) or to put Varnish or a CDN in front of your site so that you're not tying up your limited Apaches just serving static assets,
I hope that helps,