A GUID (globally unique identifier) is a 32 character hexadecimal string:
If you randomly generate 2, the chance of them being the same is incredibly small.
But what if you generate 1,000,000, what are the chances there is 1 or more duplicates in those 1,000,000?
What about 10,000,000, or 100,000,000 or even 1 billion? Each new GUID has a chance to match all those previously inserted into the set.
Thanks to Rawlings answer we have the following graphs: