|Stamp issue date|Stamp set|#cards|Card set ref|Qty. Issued|
|---|---|---|---|---|
|16 May 1973|County Cricket, 1873–1973|one|PHQ 1|20,000|
|15 August 1973|400th Anniversary, birth of Inigo Jones|one|PHQ 2|20,000|
|12 September 1973|19th Commonwealth Parliamentary Conference|one|PHQ 3|15,200|
|14 November 1973|Royal Wedding (The Princess Anne)|one|PHQ 4|290,000|
|27 February 1974|British Trees|one|PHQ 5|16,750|
|24 April 1974|Fire Service Anniversary|one|PHQ 6|30,000|
|10 July 1974|Medieval Warriors|four|PHQ 7|31,750|
|9 October 1974|Winston Churchill Centenary|one|PHQ 8|114,430|
|19 February 1975|Bicentenary, birth J. M. W. Turner|one|PHQ 9|46,250|
|23 April 1975|European Architectural Year|three|PHQ 10|51,000|
|11 June 1975|Sailing|one|PHQ 11|41,350|
|13 August 1975|150th Anniversary of Public Railways|four|PHQ 12|47,500|
|22 October 1975|Jane Austen Bicentenary|four|PHQ 13|27,330|
|28 April 1976|Social Reformers|one|PHQ 14|38,350|
|2 June 1976|American Bicentenary|one|PHQ 15|60,950|
|30 June 1976|Roses|four|PHQ 16|34,250|

Question: how many stamp sets had at least 50,000 issued?
Answer: 4