In this regard SHA-1 despite it's weaknesses can be viewed as a pseudo random function. This means we are are throwing $n$ balls into $n$ bins. An output bin remains empty if all the balls miss it. Which happens with probability $(1-1/n)^n$ which is $1/e$ and that is the portion of output bins which are empty. We also can estimate that the most populated bin has approximately $log(n)$ balls.
Meir Maor
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