The answer was here:
A group that creates 100% addresses where one of them will start with a given prefix has size
in other terms: no, there is not such a number, or if you prefer there is but it is a very very large number, something like
2^256 - (2^160/difficulty)*2^96
where (2^160 / difficulty) is the size target (number of addresses with a given prefix, 2^96 is the average number of private keys to the same address).
But nobody knows exactly how many private keys generate addresses with a given prefix, it is only a estimate!
What percentage does this number specify? (I can't find an effective calculation method myself)
difficulty = 10054102514374868992 = 2^(63.1)
number of keys we have to use (= number of addresses we have to generate to get on average 1 match)
target = 2^160 / 10054102514374868992 = 1.453637095147298e+29 = 2^(96.9)
number of addresses in target
difficulty * target = 2^160 (all addresses)!
The difficulty is how much the set of all addresses is bigger than set of the target addresses. In this case:
difficulty = set of all addresses / target set = 2^160 / 2^(96.9) = 2^(63.1)
Smaller is the target, bigger is the difficulty to hitting it and viceversa.
How to compute the probability?
The formula (probability to hit a element in the target set in n tries) is:
P(n) = 1-(1-p)^n
where p = 1/difficulty (probability to get a match each try).
In your case:
p = 9.946 * 10^(-20)
P(1) = 1-(1-p)^1 = p = 9.946 * 10^(-20)
P(100) = 1-(1-p)^100 = 9.94 * 10^(-18)
P(10054102514374868992) = 1-(1-p)^10054102514374868992 = 0.63 -> 63%
P(2*10054102514374868992) = 1-(1-p)^2*10054102514374868992 = 0.86 -> 86%
P(3*10054102514374868992) = 1-(1-p)^3*10054102514374868992 = 0.95 -> 95%
P(k*diff) = 1-(1-p)^k*diff = 1- e^-k (this way is more simple to compute)
If you want to know what is n to get P(n)=99%
--> 1-(1-p)^k*diff = 1 - e^-k = 0.99
--> e^-k = 0.01
--> k = -ln(0.01) = 4.6 --> n = 4.6 * 10054102514374868992
P(4.6*10054102514374868992) = 1-(1-p)^4.6*10054102514374868992 = 0.99 -> 99%
P(6.9*10054102514374868992) = 1-(1-p)^6.9*10054102514374868992 = 0.999 -> 99.9%
and so on.