What is right, is that no site has enough information to get a valid droprate. And of course even if it was 0.1%, it doesn't mean that if you do 1'000 runs you're guaranteed to get it. To be 100% sure you get the mount needs an infinite number of runs which is, quite frankly, impossible. What can be calculated, however, is the number of Baron kills you need to do to be 99% sure it drops. This is just for the sake of being an exaple, we do not know the actual drop percentage so we assume it's 0.1%. You can use your own droprate if you want to follow along.
So, let A be "Baron drops the mount". So P(A)=0.001, assuming 0.1% droprate. P(B)=0.999, where B is "Baron doesn't drop the mount". (no, i cant write a complement line here so it's B). The chance of Baron not having dropped the mount on x runs is 0.999^x.
Therefore,
the chance of Baron dropping the mount at least once on x attempts is 1-0.999^x.
If we want to be 99% sure it drops on these x runs, we let 1-0.999^x=0.99.
We can then solve x, so we get the number of runs needed for 99% assurance:
0.999^x=0.01
ln(0.999^x)=ln(0.01)
x*ln(0.999)=ln(0.01)
x=ln(0.01)/ln(0.999)
x=4602.867
So to be 99% sure it drops, we need about 4600 runs.