Hello Janne,
Below is a sample query I used to get the total number of test cases that are linked to specific Requirements. If you need more information in the Query please take a look at the Database reference Guide in QC, By going to help Documentation Library Then Opening the Database reference guide. This guide will need to be saved on the local machine in order to be opened. I would also seek the help of a DBA in your company to help refine the query to your specific needs. Hope this helps.

Select REQ_COVER.RC_REQ_ID /*Requirement Coverage.Requirement*/, Count(REQ_COVER.RC_ENTITY_ID) /*Requirement Coverage.Covering Entity ID*/
From REQ_COVER, TEST
Where REQ_COVER.RC_ENTITY_ID = TEST.TS_TEST_ID
Group by REQ_COVER.RC_REQ_ID