Typically this message has nothing to do with RAM and is actually a warning about GDI objects. You can verify this by opening the Windows Task Manager and clicking the Processes tab. From the View menu, click Select Columns and then add the User Objects and GDI Objects columns. If the GDI Objects value is at or above 9,000 for the SLDWORKS.exe process, you are hitting the limit where SW will start to warn you about it. Windows processes have a limit of 10,000 GDI objects.
Prior to SW 2015, I regularly would open 20 or more models/drawings and never had a problem. Starting with 2015 (and now 2016) there is some sort of GDI Object leak in SW that causes it to grow until the limit is hit, triggering the warning.