There is no retrieve subitems GraphQL API, we can retrieve subitems using retrieve item. Also, there is no pagination support for subitems. It is always retrieve all the subitems. Considering this, Is there any limit on the number of subitems for each item in the board?
If no pagination on subitems query and no limit on number of subitems can be added for particular item. How retrieve will work when one item is having many subitems like few 1000s or 100000s of subitems.
According to the documentation (and my own testing) the combined total number of items AND subitems cannot exceed 10,000.
Also, it seems that you might not have understood Matias’ comment “You can use pagination in the items() query using the board ID of a subitems board before items().”
You CAN use pagination when you query subitems… as long as you are querying them via the subitems board.