Into how many classes does BBC divide the recorded knowledge?

Bliss Bibliographic Classification (BBC) divides recorded knowledge into 21 main classes, which are further divided into subclasses. The 21 main classes are:

1. Philosophy and Psychology
2. Religion
3. Social Sciences
4. Language and Literature
5. Pure Science
6. Applied Science
7. Fine Arts
8. Practical Arts
9. Recreations
10. Generalities
11. History
12. Geography
13. Biography
14. Natural History
15. Social Sciences (continued)
16. Language and Literature (continued)
17. Science (continued)
18. Applied Science (continued)
19. Practical Arts (continued)
20. Fine Arts (continued)
21. Literature (continued)

Each of these main classes is further divided into subclasses using decimal numbers. For example, class 5 (Pure Science) is divided into subclasses 5.1-5.9, which represent different areas of science. Similarly, class 15 (Social Sciences (continued)) is divided into subclasses 15.1-15.9, which represent different areas of the social sciences.