not the best design to have 1000 rows for each shop*SKU combination. If a candidate proposed this solution during Shopify's System Design interview, i doubt he would be vetted for Senior+ position.

Instead of having 1000 rows per shop*SKU, why not just have one row per shopping cart*SKU?

That way a single row would represent a single cart, and will hold info of multiple items of the same SKU.

No need a cludge with 1000 rows limit and replenishment process. Instead of dealing with N rows, you always deal with a single row.