Actually i read the comparisons between the standard and pro Version on the parallels website.
There is one comparison named: "Run professional, graphics-heavy windows applications and multiple virtual machines." The table says: Standard version -> no check box set, Pro Version-> checkbox set.
I´m unsure now. I actually use standard version 17 and can run multiple vm´s (have enough RAM) with no problem. I found no information about this on the website and there´s no official mail-support.
Is there an official statement about this