That means is at least 400 2nd stages built for the Falcon family of rockets. With over 100 last year alone. From a production & QC stand point that is very impressive. Other companies are trying to grab a share of SX's launch market but being able to produce that many rocket engines and stages is very impressive.
It also shows the real economic impact how how a completely reusable system will change the game. I guess the cost of the 2nd stage is becoming a larger part of the real cost for SX to launch Falcons when you are able to get reuse up to around 10+ flights for the 1st stage.