As of December 2016, there were 2.2 million top level IDNs and 6.5 million second level IDNs.
Top Level IDNs
Top Level (or full) IDNs have the same script at both the TLD level and the domain label.
Second Level IDNs
Second level IDNs have a non-ASCII label under an ASCII TLD.
Second level IDNs have been available since 2000, both in the ccTLD and gTLD spaces. Latin script IDNs (eg with accents and diacritics) have a natural affinity with ASCII TLDs such as .com or .eu.
It was not possible to register full ccTLD IDNs until 2009, when IDN strings were allowed into the root IANA database through the ccTLD IDN Fast Track process.
At the gTLD level, gTLD operators had to wait for ICANN to open the call for a new gTLD round to be empowered to submit applications for IDN TLDs, which happened only three years ago, hence the low percentage (1%) of top level IDNs in the gTLD space.
In December 2015, 67% of registered IDNs were at the second level, and 33% were full IDNs. During 2016, the growth of second level IDNs under .cn increased the proportion of second level IDNs to 75%, compared with 25% at the top level.