A cap will be applied to the Sellable_Quantity field in the Product API June 1, 2021. The cap has been set to 25. If it should change, Partners will be notified.
If a product has more units available for sale than the Sellable_Quantity cap, the cap value will be displayed (currently 25). The Sellable_Quantity field will show the true inventory level once the unit count drops below the cap.