Go to Parallels Desktop -> Preferences -> Keyboard. There you can define what keys on the Mac go to which keys on the PC.
The F keys go up to F19. If you can't get F20-F24 then you could try modifying the keyboard profile files manually. They are xml files with .kbd file name extensions in ~/Library/Preferences/Parallels.
Parallels key codes are defined in:
"/Library/Parallels/Parallels Service.app/Contents/Frameworks/ParallelsVirtualizationSDK.framework/Versions/6/Headers/PrlKeys.h"
For example, F19 is 158 so you would see it in the .kbd file as <CustomKey>158</CustomKey>