It is difficult to quantify how many NxT users exist.
On the Nxt forum there is a stat that 509 was the most online ever , but because of the severe decrease in volume and market cap adoption may have reversed. I would guess there is at least 500 - 2000 Nxt investors now with 7-20 with very large purses. Most of these rarely use the currency but that is enough of a userbase to insure NxT stays around for many years. Whether or not NxT will continue to slide in crypto-currency marketshare is another story however. I would suggest that they likely will become less relevant and even bitcoin will face some stiff competition from certain government and corporate blockchain currencies about to come out.

We definitely have some interesting years ahead.

P.S.. Are there any regulated NxT exchanges with full KYC to give use a better idea of user adoption? BTC has coinbase where we can estimate over 4+ million based upon their KYC and the fact that the largest adoption is happening outside of their jurisdiction.