After a number, or after several or a few, use the singular form thousand:
✗ There are about fourteen thousands airports all over the world.
✓ There are about fourteen thousand airports all over the world.
✗ Several thousands residents have to be moved to allow the construction to proceed.
✓ Several thousand residents have to be moved to allow the construction to proceed.
When you are not giving an exact number, you can use the plural form thousands with the preposition of:
Every day, thousands of tons of waste were sent to landfills.
The words hundred and million work in the same way.