In Hebrew you write the amount of things according to their gender. 16 can be שש עשרה (fem.) or שישה עשר (masc.). Since hat (כובע) is a masculine noun you need to say שישה עשר.
Btw what you wrote (שיש עשרי) is incorrect spelling in Hebrew , you probably meant שש עשרה (fem.) since it probably sounds the same.
Hope it helps :)