System administrators can assign dedicated RAM to a VPS to make sure the user always gets the amount promised. But the real question is: how much do you really need? Dedicated servers may have 2GB, 8GB, or even 64GB of RAM, depending on the tasks they perform. Some high-powered servers have so much memory, they stop calculating it in gigabytes. For a VPS, however, you may have as little as 128MB of RAM dedicated to your virtual server.
Average VPS allocations typically run in the 512MB to 2GB range, and the amount you actually need depends on what you use the server for, the amount of traffic it receives, and how efficiently your OS and software use it. For example, a server that runs Apache by itself will use far less memory than one also running MySQL. Web applications that are dynamically called every time someone accesses a website will require more memory than static HTML pages. If you use caching techniques, you can often reduce memory usage.
As your website grows, you will likely need to increase the RAM allocated to your VPS and possibly move onto a dedicated server eventually.