FYI, DPI is a term from the print world; it stands for Dots Per Inch. It pretty much translates precisely to the digital world of PPI, Pixels Per Inch.
Super 35mm film is 24.89 mm by 18.66 mm (0.980 in by 0.735 in); if your client wants 1200 DPI/PPI, all you need to do is the math. It works out to be 1176px by 882px.
I’d double-check with the client and confirm that’s what they want. They might have an actual dimension in mind, such as 10 inches on the long side. If that’s the case, then the DPI/PPI for an image file scanned from a Super 35 frame to make a print 10 inches on the long side would be 12000px by 9000px.
1200 DPI/PPI is an odd request for print as the industry standard is 300 DPI/PPI.
You mention 2K in your post. If you are scanning Super 35mm to standard video 2K resolution (1920px on the long side), your numbers would be 1920px by 1440px.