Do more desktops occupy more ram?
I've never seen a single instance here I could pin a performance issue on Mission Control having 5 spaces instead of 15 instead of one.

Something else always seems to be the constraint such as RAM pressure or IO pressure. The code that manages the window system already treats each layer virtually and the math is done in some heavily optimized routines and the data structures there are quite well designed so although in theory - tracking more pixels could be a problem for performance, in reality it isn't.



If you are skeptical, you can reboot your Mac and do this test:


Open whatever set of Apps and documents you want to test with.
Enter Mission control and delete all the spaces except for one.
Open terminal and run vm_stat 60
Watch for a few minutes to be sure all the running processes are done allocating RAM. Switch to each App and hide or show the windows you want for your test. Once the free and active numbers are steady - open mission control and leave it open for a minute or two - then go back and inspect RAM allocations.
Add another 10 spaces and see how RAM allocations change.
Move app windows to any desktop and verify that no significant RAM allocation changes happen when you are using Mission Control and that they only happen when an app opens a new document or window. 


Yes - if you tend to keep more things open when you have more "virtual space", that takes more RAM, but the fact that you have X apps and Y windows open is what takes the RAM - not which of Z mission control virtual screens you have open at any one time.