“Only 2 left in stock — order soon!”
You're on an online shopping site. You're looking at a listing for a book that's on sale. In the information next to the item, there's a sentence which says this.
Only 2 left in stock — order soon!
When a store or company has enough of something to sell, you say that the item is "in stock". So "in stock" means "available to be sold".
The opposite of "in stock" is "out of stock". On a shopping website, if an item has been completely sold out, there will be a message saying:
Temporarily out of stock.
When almost all of something has been sold, you can express how many there still are with the phrase "___ left in stock":
We only have one left in stock.
Let me check with our other store to see if they have any left in stock.