The File Number starts with a letter followed by 9 numbers. eg. F000000000.
UCI number has eight numbers, and no letters
You will find both at the top of the letter usually under the dear sir/madam..
the link below shows each number, and where to find it.
https://services3.cic.gc.ca/ecas/redir.do?lang=en&redir=id_num&_ga=1.174884578.1108105437.1420929627