The number of partitions is fixed at the time of the index creation. Each partition is an indivisible chunk of data which can grow in size but not be split into smaller chunks. As the data grows, more nodes may be added to the cluster, but the number of partitions which eventually move to the newer nodes are limited (Fig 3)
Targeting back-to-back Falcon 9 launches of @Starlink satellites to low-Earth orbit from both Florida and California starting on Wednesday, August 16 → http://spacex.com/launches