MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 13: Descriptors

Record Count

The SQL_DESC_COUNT header field of a descriptor is the one-based index of the highest-numbered record that contains data. This field is not a count of all columns or parameters that are bound. When a descriptor is allocated, the initial value of SQL_DESC_COUNT is 0.

The driver takes any action necessary to allocate and maintain whatever storage it requires to hold descriptor information. The application does not explicitly specify the size of a descriptor nor allocate new records. When the application provides information for a descriptor record whose number is higher than the value of SQL_DESC_COUNT, the driver automatically increases SQL_DESC_COUNT. When the application unbinds the highest-numbered descriptor record, the driver automatically decreases SQL_DESC_COUNT to contain the number of the highest remaining bound record.