These are the National Parks I've visited so far.
42/63 National Parks = 21 to Visit
1. Acadia National Park
2. Arches National Park
3. Badlands National Park
4. Big Bend National Park
5. Biscayne National Park
6. Black Canyon of the Gunnison National Park
7. Bryce Canyon National Park
8. Canyonlands National Park
9. Capitol Reef National Park
10. Carlsbad Caverns National Park
11. Channel Islands National Park
12. Congaree National Park
13. Crater Lake National Park
14. Cuyahoga Valley National Park
15. Death Valley National Park
16. Dry Tortugas National Park
17. Everglades National Park
18. Gates of the Arctic National Park and Preserve
19. Gateway Arch National Park
20. Glacier Bay National Park and Preserve
21. Glacier National Park
22. Grand Canyon National Park
23. Grand Teton National Park
24. Great Basin National Park
25. Great Sand Dunes National Park and Preserve
26. Great Smoky Mountains National Park
27. Guadalupe Mountains National Park
28. Haleakala National Park
29. Hawaii Volcanoes National Park
30. Hot Springs National Park
31. Indiana Dunes National Park
32. Isle Royale National Park
33. Joshua Tree National Park
34. Katmai National Park and Preserve
35. Kenai Fjords National Park
36. Kings Canyon National Park
37. Kobuk Valley National Park
38. Lake Clark National Park and Preserve
39. Lassen Volcanic National Park
40. Mammoth Cave National Park
41. Mesa Verde National Park
42. Mount Rainier National Park
43. National Park of American Samoa
44. New River Gorge National Park and Preserve
45. North Cascades National Park
46. Olympic National Park
47. Petrified Forest National Park
48. Pinnacles National Park
49. Redwood National Park
50. Rocky Mountain National Park
51. Saguaro National Park
52. Sequoia National Park
53. Shenandoah National Park
54. Theodore Roosevelt National Park
55. Virgin Islands National Park
56. Voyageurs National Park
57. White Sands National Park
58. Wind Cave National Park
59. Wrangell-Saint Elias National Park and Preserve
60. Yellowstone National Park
61. Yosemite National Park
62. Zion National Park
63. Denali National Park and Preserve