How many unencoded characters still exist?
GlyphWiki.org contains 580’000 characters. 92’856 are encoded. You can do the math. However, it will be wrong, as most of these characters are trivial alterations of those already encoded, and thus ineligible for a separate encoding.

So, instead let’s look at the pipeline of next encodings. Extension I (officially, Working Set-2017) is being finalized right now (and can be browsed at https://hc.jsecs.org/irg/ws2017/app/) to around 5’000 characters. Among these:


PRC has hundreds to offer, many of these Zhuang;
Taiwan is almost exhausted and sends only a small number of chemical characters;
ROK has hundreds, but much smaller number, of historical characters;
Vietnam still has some Nom;
However, the UK/UTC selection of scholars is still going strong, providing, say, data from dialect dictionaries.
To say nothing of Buddhist characters.


Most importantly, ROK and Vietnam offerings are approaching conclusion, and it is unlikely there will be substantial extensions thereof. Unless some new surnames or geographical locations are discovered, Japan and Hong Kong have no more intention for encoding.

I expect the following encodings after I to be in number of thousands, not tens of thousands, and the scholarly UK/UTC extensions be the lion’s part of it.