how person, can open my website at same time if i have JSm2...

reduce the use of ram this program in a virtual server?

The person that access you website will do it as usual.
They will arrive to the same Joomla and this is your web server that will manage the connection. So in terms of memory, this will be the same as you have X user that connect the same URL. If you define several Virtual Server, the result will be the same.
The particularity of JMS is that it is able to select the appropriate Joomla configuration to retreive to connect the database and retreive the context for the website rendering.
Up to you to define on Virtual Server with Alias server or Define as many Virtual Server as you have domain or sub-domain.
If you web server is able to optimize the memory in case where you have a single Virtual Server with several aliases, in this case, perhaps you will reduce the memory used.
Otherwise, I don't think you will reduce the RAM used.