MySQLi identifying multiple queries on same object

Hi,

I recently migrated to mysqli after reading the old type is going to be phased out, but I dont understand how you identify queries:

//Assume $db is already instantiated 

$db->query("SELECT xyz FROM abc....."); //Query 1 
$db->query("SELECT efg FROM def....."); //Query 2 
$db->query("SELECT pqr FROM hij....."); //Query 3 

//Here I want to see how many rows are in query 1 and return an array for query 2

How do I tell the $db->num_rows property what query to look at as it does not have any parameters?

...and the same goes with fetching arrays and so fourth; all of these methods do not take any parameters regarding what query they are using. As far as I can see the only way would be to have one query per $db object which can't be true.

Thanks.