Number of Queries:

Two categories of query structures can be used for a matrix data model.

One-Query Matrix

A matrix report could be created from a single query where a single SQL Select statement is used to fetch data from the database.  The Reports Builder toll then groups the data fetched from the database to construct a matrix report.

Multi-Query Matrix

Multi-query data model can be considered since it results into easy queries as compared to one query data model.  Additionally, a multi-query data model is needed for nested matrix reports.