You don't need to simulate, it is a well studied problem [1]

It seems that the average number of expected collisions for 537k would be 0.13 and not 1:

the average value of pairs of individuals with the same birthday (or hash collisions) is E[X] = k*(k-1)/(2*n). Plugging in k=2^40 for 40 bits and n=537e3 I get their result of 0.1311

Thanks for the nerdsnipe, I ran out of my daily LLM mistake fixing quota !

[1] https://en.wikipedia.org/wiki/Birthday_problem#Average_numbe...