I decided to look back to see what the numbers of active members were 10 and 20 years ago:
Jan 2009: 13 (excluding the Cambridges, Sussexes and adding the duke of Edinburgh & duchess of Kent)
Jan 1999: 11-12 (excluding Camilla, the Wessexes, the Cambridges, the Sussexes; and adding the duke of Edinburgh, duchess of Kent, Queen-mother, princess Margaret; I am not sure how active the Queen Mother was aged 99)
Going back to the start of her reign:
The queen started with 7 active members (not counting queen Mary who passed away a year later): herself, the duke of Edinburgh, the queen-mother, the princess Margaret and the duke and duchess of Gloucester and the dowager duchess of Kent as active members.
She purposefully enlarged that number to include 3 more members in the next 10 years, raising the number to about 10 (as expected
the duke of Kent and therefore also the duchess of Kent, and (not-necessarily expected
princess Alexandra.
.