Maximum Number of Relations

[CLIFORD JOSHY]

I was reading through NITCbase for learning purposes and saw the line,
"Q2. What is the minimum number of entries allowed in the Relation Catalog? What relations do these entries correspond to?

The minimum number of entries in the Relation Catalog is two, even in the absence of other relations in the database. These entries correspond to Relation Catalog and Attribute Catalog since they are also treated as relations and should be in the disk at all times. Thus, the maximum number of relations allowed in NITCbase is 18 (why?)."

Shouldn't the maximum no of relations be 19, as 2048/96 is 21 and 2 relations are taken up by relation catalog and attribute catalog. I don't quite understand how the number 18 came as the answer.

[CLIFORD JOSHY]

  There is a header for the block that takes up 52 bytes.  This is why the number of catalog records possible is one less than 2048/96 - that is 20. By reserving entries for the two catalogs, the number of user defined tables are restricted to 18.

  Please have a look at the figure in the documentation link: https://nitcbase.github.io/docs/Design/Physical%20Layer/

this query was mirrored from an informal email conversation for documentation purposes