CREATE TABLE table_28235 ("NSG Nr." FLOAT, "Name of the nature reserve" TEXT, "District / Town" TEXT, "Area  ( ha ) " TEXT, "Date established" TEXT)

What Nature reserve number was established on 19961220 20.12.1996
SELECT MIN("NSG Nr.") FROM table_28235 WHERE "Date established" = '19961220 20.12.1996'
SELECT MIN("NSG Nr.") FROM "table_28235" WHERE "Date established" = '19961220 20.12.1996'