Hi Anis,
First, as I see from your account, you have a transmuxing from RTMP to HLS (as in http://blog.wmspanel.com/2014/06/rtmp-hls-transmuxing.html), not RTMP republishing (as in http://blog.wmspanel.com/2014/10/rtmp-republishing-nimble-streamer.html)
Currently Nimble uses about 130 MB of RAM for caching and operational purposes, the rest is taken by the OS. The transmuxing of each stream needs an amount of RAM equal to stream's bitrate multiplied by 30 seconds (default chunk cache lifetime). Summing all streams, you'll get cache size. Plus, Nimble takes a few MBs for operational purposes.
You can run top or htop command to see how much is consumed by Nimble, by OS and by other services.
Please notice that it's a public resource, so I cannot expose much information about your account usage, so if you'd like to get some specific help about Nimble Streamer usage or fine tuning, please contact our support team.
Thank you.