Random string probability calculation

I am using the RNGCryptoServiceProvider to generate a 256-bit length random string (not number), from the characters a-z/A-Z/0-9.

I am required to generate some statistics with regards to the feasibility of collisions (what is the chance, and how many could occur) when generating tokens per second but am slightly struggling to show this in a good way?

Is there a formula already used for working out such probabilities?

I had a look around at similar for SHA-256 calculations but I am not sure if it fits this model exactly.

Any help would be greatly appreciated