CREATE TABLE table_26013618_1 (nsg_nr INT, date_established VARCHAR)

What Nature reserve number was established on 19961220 20.12.1996
SELECT MIN(nsg_nr) FROM table_26013618_1 WHERE date_established = "19961220 20.12.1996"
SELECT MIN("nsg_nr") FROM "table_26013618_1" WHERE "19961220 20.12.1996" = "date_established"