The CiCurrentRecordNumber built-in variable contains the record number of the current record (1-based). The record number starts at [(page-number - 1) * records-per-page] + 1 for any page. Each time through the <%begindetail%> section, the value of CiCurrentRecordNumber increases by one.
The CiMatchedRecordCount built-in variable contains the value of the total number of records matched in the query (limited to CiMaxRecordsInResultSet). The CiRecordsNextPage built-in variable contains the number of records that will appear on the next page. Referring to either CiMatchedRecordCount or CiRecordsNextPage will cause the query to be nonsequential.