Why is the number of pages that Google is indexing fluctuates greatly?

it is a wholesale page

This would be for me an explanation of such ups and downs.

First of all, not all elements are unique, so Google needs time to get this.

Secondly, I suppose, it has a high volatility in its site: the products are published and sold out very quickly.

Indexing is a slow process: it happens very often, right after your product has been indexed, it is already out of stock and Google tries to get the updated status through the de-indexing of soldout products again.

This configuration is not dangerous at all. But it is not optimal in terms of the use of Google resources. Try to index only those products, which are always for sale. Another good way could be to index product categories.