You don't have to worry about that unless you are planning on running a lot of large programs all at once.
MACOS is a page based memory model. These pages are small (512KB) and allocated to programs as needed, and deallocated when not being actively used and another program needs memory. Also if parallels, or another program runs, and needs more memory and all the memory on your system is being use, the operating system will make other programs give up some of your memory to meet the needs of parallels.
However, a problem can occur when if you have a lot of programs and they all need a lot of memory, AT THE SAME TIME. But, with a 16GB system we are talking about several large programs like Photoshop all performing operations at the same time. And if that were the case you would likely run into other issues like lack of CPU cycles before running out of memory.
So if you could tell us what you plan on doing with your machine, people could evaluate the likelihood of you having memory issues.