How many unique URL's can goo.gl support?

How many URL's can goo gl support? And what is the current rate at which it is being filled up. I can see that it creates unique URL's using small and big alphabets and n开发者_JS百科umbers. That gives it a capacity of (26*2+10)^5, is this estimate correct.

I think the math is correct.. but it's unlimited. Even if the current 5 letter space is used up they can just add another letter.