What this tile shows
Counted from your own order items, not guessed. It reports pairs that genuinely recurred, so it is evidence for a bundle rather than a recommendation engine.
Squizzie calls this one Frequently Bought Together. It is a small table rather than a single figure, so it carries a few rows of detail on the tile itself.
Where the number comes from
WooCommerce Products. Product performance and inventory data.
What you need for it to appear
Nothing beyond Squizzie and the shop data it already reads.
The fuller picture
This is one figure out of a bigger subject. Which products actually get bought together covers it properly.