According to APNIC labs, IPv6 adoption in India is ~79% and in China it is ~53%.

Those are the only two countries that could plausibly have billions of mobile devices and they appear to have reached 50%.

India: https://stats.labs.apnic.net/ipv6/CN?c=IN&x=1&v=1&p=1&r=1&w=...

China: https://stats.labs.apnic.net/ipv6/CN?c=CN&x=1&v=1&p=1&r=1&w=...