Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Serial number | River | Name | Area (km²) | District | Co-ordinate | Altitude (m) |
| --- | --- | --- | --- | --- | --- | --- |
| 1. | Bharathapuzha | Malampuzha | 23.13 | Palakkad | 10°50′N 76°41′E﻿ / ﻿10.84°N 76.69°E | 104 |
| 2. | Bharathapuzha | Mangalam | 3.93 | Palakkad | 10°31′N 76°32′E﻿ / ﻿10.51°N 76.54°E | 72 |
| 3. | Bharathapuzha | Meenkara | 2.59 | Palakkad | 10°37′N 76°48′E﻿ / ﻿10.62°N 76.80°E | 152 |
| 4. | Bharathapuzha | Chulliar | 1.59 | Palakkad | 10°35′N 76°46′E﻿ / ﻿10.59°N 76.77°E | 143 |
| 5. | Bharathapuzha | Pothundi | 3.63 | Palakkad | 10°32′N 76°38′E﻿ / ﻿10.54°N 76.63°E | 93 |
| 6. | Bharathapuzha | Walayar | 2.59 | Palakkad | 10°50′N 76°52′E﻿ / ﻿10.84°N 76.86°E | 197 |
| 7. | Bharathapuzha | Kanjirampuzha | 5.12 | Palakkad | 10°59′N 76°33′E﻿ / ﻿10.98°N 76.55°E | 90 |
| 8. | Chalakkudy | Parambikulam | 20.92 | Palakkad | 10°23′N 76°48′E﻿ / ﻿10.39°N 76.8°E | 545 |
| 9. | Chalakkudy | Thunakkadavu | 2.83 | Palakkad | 10°25′59″N 76°47′02″E﻿ / ﻿10.433°N 76.784°E | 565 |
| 10. | Chalakkudy | Peruvaaripallam |  | Palakkad | 10°26′49″N 76°46′12″E﻿ / ﻿10.447°N 76.77°E | 565 |
| 11. | Chalakkudy | Sholayar Dam | 8.70 | Thrissur | 10° 17' 76° 45' |  |
| 12. | Chalakkudy | Peringalkuthu Dam | 2.63 | Thrissur |  |  |
| 13. | Kallada | Parappar | 25.90 | Kollam | 09° 57' 77° 4'20" |  |
| 14. | Karamana | Aruvikkara | 2.58 | Thiruvananthapuram | 08° 28' 77° 58' |  |
| 15. | Karuvanoor | Peechi | 12.63 | Thrissur | 10°32′N 76°23′E﻿ / ﻿10.53°N 76.39°E | 73 |
| 16. | Keecheri | Vazhani | 2.55 | Thrissur | 10° 40' 76° 15' |  |
| 17. | Kuttiadi | Kuttiadi | 10.52 | Kozhikode | 11° 36' 75° 49'27" |  |
| 18. | Neyyar | Neyyar | 15.00 | Thiruvananthapuram | 08° 32' 77° 08' |  |
| 19. | Pamba | Pamba | 5.70 | Pathanamthitta | 09° 20' 76° 53' |  |
| 20. | Pamba | Kakki | 18.00 | Pathanamthitta |  |  |
| 21. | Periyar | Idukki | 61.60 | Idukki | 09° 48' 76° 53' | 720 |
| 22. | Periyar | Ponmudi | 2.60 | Idukki | 09° 55' 77° 05' |  |
| 23. | Periyar | Anayirankal | 4.33 | Idukki | 10° 0' 77° 0' |  |
| 24. | Periyar | Kundala | 2.30 | Idukki | 10° 0' 77° 0' |  |
| 25. | Periyar | Mattupatti | 3.24 | Idukki | 10° 05' 77° 05' |  |
| 26. | Periyar | Sengulam | 0.33 | Idukki | 10° 00' 77° 05' |  |
| 27. | Periyar | Neriamangalam | 4.13 | Idukki |  |  |
| 28. | Periyar | Bhoothathankettu | 6.08 | Idukki |  |  |
| 29. | Periyar | Periyar lake | 28.90 | Idukki | 10° 10' 76° 15' |  |
| 30. | Valapattanam | Pazhassi | 6.48 | Kannur |  |  |
| 31. | Vamanapuram | Peppara | 5.82 | Thiruvananthapuram |  |  |
| 32. | Muvattupuzhayar | Malankara | 11.00 | Idukki |  |  |
| 33. | Chimmini dam | Thrissur | 85.067 | Thrissur |  |  |
Question:
how many have an area of at least 20 km?
Answer:
6