Go to Product Page > Sections > Product Summary
You can show these option:
Number of words that will be displayed for each product content if you use 'Excerpt' mode and don't set custom excerpt for each product content.
You show it by using the icon or image
9.1.1 - Default:
Show when less than (X) items are in stock
9.1.2 - Random
It shows random quantity. It's fake quantity, not real.
9.1.3 - Default + random
If the quantity in stock product <= (X) ---> it run as default.
If not, it runs as random