db_query() returns a DatabaseStatementInterface object which has a function for retrieving the number of returned rows in the last SQL statement.

You could use the following piece of code to get the total number of nodes in Drupal.

$num_of_nodes = db_query('SELECT nid FROM {node}')->rowCount(); 

Done =)

Reference:

• Druapl API – db_query
• Drupal API – DatabaseStatementInterface
• Drupal API – DatabaseStatementInterface::rowCount