I want to enable port 8888 so that somebody can access my files, should I set the Virtual Server 'Service Port Range' to 8777 ~ 8779 or should I set it to 8888 ~ 8888? I don't want to leave any more ports open than necessary.

If you want to open only a specific port, then you should enter 8888~8888, this will open only one port 8888. If you want to open a range, then you should enter 8770~8779, this will open ten ports from 8770 to 8779.