There are now close to 17,000 clusters (2003) worldwide, out of which 500 clusters have crossed 3rd milestone (in 2019)

"Reports received at the World Centre indicate that there are now close to 17,000 clusters worldwide, excluding those countries where, for one reason or another, the operation of the Faith is restricted."

-The Universal House of Justice, 17 January 2003

https://www.bahai.org/r/532027753