Limitations of the free version
The free version of the app is limited to 1000 Shopify products – including ALL their variants
On your shop you are selling 1000 different T-Shirts as products, each comes in two sizes and two colors. That would make 1000 × 2 × 2 = 4000 variants and thus your feed will contain 4000 Facebook products/items.
That is why even though the number of products is limited the actual number of items in the feed can vary widely.
For shops with a lot of variants per product the feed can grow very large very quickly.
Overcoming the Limits!
You can work around the limit and achieve unlimited number of products in your Facebook product catalog. To do this you need use a (premium) feature called feed pagination.