For a large e-commerce project we need to import more than 20 000 documents like CMS pages (not products) and probably maintain them in the future. Does Prismic has a feature to import this data possibly through an API or any other simpler way to the import? Currently, manually importing and maintaining so many documents by using Prismic UI with 200 documents batch is counterproductive.
Prismic does not offer any kind of document write API at the moment. The only option right now is the import/export feature, but as you've stated above it's limited to 200 documents at a time.
This is being tracked as a feature request here: