Creationists love to spout completely made-up numbers to show how "impossible" it was for life to appear on its own (or whatever else they are trying to prove.) In some cases there might be some semblance of actual estimation, but the problem even in these cases is that they make completely wrong assumptions.
For example, they may ask "what's the probability that all the atoms needed to form a single DNA molecule just happened to be in the right place at the right time?" Then they make up some figure.
The major problem here is, of course, that they assume that the original DNA molecule formed instantaneously, rather than it having been a very gradual process of "proto-DNAs" changing over time. However, an even more fundamental problem with this is that they assume that atoms just float around randomly, with no natural processes affecting them, and thus they coming together is just pure chance.
Even an elementary school textbook will tell you that atoms don't just float around without anything affecting them. They interact with each other, they attract or repel each other, they form atomic bonds, they form ionic bonds... There are all kinds of physical phenomena affecting how they interact and combine. For instance, two oxygen atoms making a bond and forming an O2 molecule is far from being a "random chance." There are physical phenomena that affect them and make this formation very likely.
To illustrate, suppose we simplify the problem a bit and ask the question: "What's the probability that two oxygen atoms and one carbon atom just happen to come together and form a carbon dioxide molecule?" If we do what creationists do, we just ignore all the natural laws governing their behavior, and simply estimate how many oxygen and carbon atoms there are on Earth, what the size of the Earth is, and from that come up with a number that tells us how likely it is for three of them to just happen to be close enough together to form a CO2 molecule. I haven't even attempted to do this calculation because it's an exercise in futility, but I wouldn't be surprised that it's like one in millions, or even trillions.
If we took that at face value we would have to conclude that the formation of CO2 is so incredibly small that there should exist none on Earth. Yet there is a lot of it.
The reason why there is a lot of it is because CO2 doesn't just form by "random chance", by three certain atoms just happening by chance to be in the right place at the right time. There are natural processes, well understood ones, that cause CO2 to form. (For example, up to 40% of the gas emitted by some volcanoes during subaerial eruptions is carbon dioxide. And we understand the reasons for this.)
Granted, the formation of organic molecules is much more complex and a significantly rarer occurrence. However, it didn't hapen "purely by chance." There are natural phenomena affecting how atoms and molecules interact with each other.