I'm assuming SHA is perfectly uniform.

In bitcoin, there are $2^{32}$ possible inputs per block, which are mapped to $2^{256}$ possible outputs. So there is a $1/2^{224}$ chance that any randomly picked 256-bit string is a possible output hash of one of the possible nonce values. So that means there are $2^{32}$ possible output 256-bit strings. In other words, **a 1-to-1 mapping from nonce to output is very likely.**

The $2^{32}$ possible outputs are distributed uniformly. So that means half of them have a 0 in the first bit, and the other half have a 1. That means there are $2^{31}$ possible outputs with a 0 in the first bit. Each leading 0 bit reduces the number possible outputs by half. So with 76 leading zero bits (the current difficulty), there's a $1/2^{44}$ chance that there exists a acceptable nonce.

Another way to see it is that there is a $1/2^{76}$ chance that any nonce is accepted. But there are $2^{32}$ possible nonce values. So the probability that none of those nonces are acceptable would be $(1-1/2^{76})^{32}$ which is something like 99.9999999999999999999%.

Either way you look at it, there's an extremely small chance that any 32-bit nonce will work.

So what happens if there are no acceptable nonces?

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