Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| 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