That seems to be the key question here, so hopefully someone with more insight can answer that.
The Delta name for the Indian variant is from the new Greek letter naming scheme.
Four out of 24 letters are in use so far:
https://www.bbc.com/news/world-57308592
UPDATE: A total of 10 letters are now in use according to WHO
https://www.who.int/en/activities/tracking-SARS-CoV-2-varian...