user: Question: How many different dates of issue are the for the coin with Kumsusan Memorial Palace on the obverse?
Columns: ['Value', 'Dimensions', 'Main Color', 'Obverse', 'Reverse', 'Watermark', 'Date of issue']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['₩10', '136 × 65mm', 'Brown-orange', 'Factory worker, Chollima (1000 Ri horse)', 'Western sea barrage and locks at Taedong Gang', 'Chollima statue', '1992, 1998']
SQL:

assistant: SELECT COUNT("Date of issue") FROM "Table" WHERE "Obverse" = 'Kumsusan Memorial Palace';