In Turkey a tax number (vergi numarasi) is required when you need to have a bank transaction done. You will need a tax number if you wish to open a bank account in Turkey.
Obtaining a tax number is not a complicated procedure. You have to go to a tax office with the photocopies of your passport. Check the following link for the tax offices in Istanbul:
http://www.ivdb.gov.tr/birimvergidairelist.htm
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