Passage: At its lower levels and at every pier, the interior of the mosque is lined with more than 20,000 handmade İznik style ceramic tiles, made at İznik (the ancient Nicaea) in more than fifty different tulip designs. The tiles at lower levels are traditional in design, while at gallery level their design becomes flamboyant with representations of flowers, fruit and cypresses. The tiles were made under the supervision of the Iznik master. The price to be paid for each tile was fixed by the sultan's decree, while tile prices in general increased over time. As a result, the quality of the tiles used in the building decreased gradually.The upper levels of the interior are dominated by blue paint. More than 200 stained glass windows with intricate designs admit natural light, today assisted by chandeliers. On the chandeliers, ostrich eggs are found that were meant to avoid cobwebs inside the mosque by repelling spiders. The decorations include verses from the Qur'an, many of them made by Seyyid Kasim Gubari, regarded as the greatest calligrapher of his time. The floors are covered with carpets, which are donated by the faithful and are regularly replaced as they wear out. The many spacious windows confer a spacious impression. The casements at floor level are decorated with opus sectile. Each exedra has five windows, some of which are blind. Each semi-dome has 14 windows and the central dome 28 (four of which are blind). The coloured glass for the windows was a gift of the Signoria of Venice to the sultan.

Question: how many ceramic tiles are handmade?
20,000