I've been wondering: How many unencoded characters still exist?
I realize this is probably mostly speculation and it would be quite hard to pin down an actual number. Language Log, though, did post a blog entitled, How many more Chinese characters are needed?, in the article they write:
This month, the Chinese government plans to introduce codes for some 3,000 Chinese characters as part of a grand project, known as the China Font Bank, to digitize 500,000 characters previously unavailable in electronic form. Until now, only 80,388 characters have been encoded in the international computing standard, Unicode.
But the comments mention a few issues with what was quoted in the NYT opinion piece. Endymion Wilkinson also broke down the five k:
It aims to lay down a more solid basis for the study of Chinese characters by encoding about 100,000 ancient Chinese scripts characters; 300,000 kaiti forms; and 100,000 minority scripts characters to give a grand total of 500,000