Not sure how you missed the electrical engineering section, but this is a VERY simple calculation. The ratio of turns from the primary side to the secondary side can be found by just dividing the primary side voltage by the desired secondary side voltage (in your case, 220v / 12v = 18.33).
To be safe, do not exceed 1 Tesla in the transformer core with no load. My experience is (without doing the math again) 1 volt per primary turn at 60 Hz if your core has a 4 square inch cross section, or 0.25 volts per square inch. This would mean 880 primary turns if your core is 1 square inch. This comes from Faraday's Law.