Read this and answer the question. If the question is unanswerable, say "unanswerable".

In-mid 2015, China started the build-up of the third generation BeiDou system (BDS-3) in the global coverage constellation. The first BDS-3 satellite was launched 30 September 2015. As of March 2016, 4 BDS-3 in-orbit validation satellites have been launched.

How many BDS-20 satellites have been launched?
unanswerable