What is the missing number?

Twenty-one reasons why English is hard to learn.
1. The bandage was wound around the wound.
2. The farm was used to produce produce.
3. The dump was so full it had to refuse more refuse.
4. We must polish the Polish furniture.
5. He could lead if he would get the lead out.
6. The soldier decided to desert his dessert in the desert.
7. Since there was no time like the present, he thought it was time to present the present.
8. A bass was painted on the head of the bass drum.
9. When shot at, the dove dove into the bushes.
10. I did not object to the object.
11. The insurance was invalid for the invalid.
12. There was a row among the oarsmen on how to row.
13. They were too close to the door to close it.
14. The buck does funny things when does are present.
15. A seamstress and a sewer fell down into a sewer line.
16. To help with planting, the farmer taught his sow to sow.
17. The wind was too strong to wind the sail.
18. After a number of injections my jaw got number.
19. Upon seeing the tear in the painting I shed a tear.
20. I had to subject the subject to a series of tests.
21. How can I intimate this to my most intimate friend?