For example the probability closest to 100 % or the number closest to infinity, does that have a representation that I can write?
I suppose it would be ∞-1/∞ ?
For example, it happens that somebody says that the probablity is 99.9999999999....%
Similarly, we have the concept of infinity in calculus, a infinitely large quantatiy or number, isn't there an expression for the smallest quantity or the smallest number so that I could more formally express the idea?
It sounds like you're looking for a "smallest positive number (in a context with subtraction)", and there are three interpretations that come to mind:
1. Is there a smallest positive real number?
No. For the real numbers, and indeed any other context (even weird ones with "infinitesimals") where you can divide and talk about "positive" numbers, you can always divide a positive number by $1+1$ to get a smaller positive number.
2. Is there a smallest positive number in any context with subtraction?
Yes. As Ross Millikan pointed out in a comment, $1$ is the smallest positive integer, so you can look at things like "$5-1$ is the integer right before $5$". Since you mentioned 100%, I doubt this case will be satisfying to you.
3. Is there a smallest positive number in any more interesting context where there are fractions like 1/2?
Yes. But it's really esoteric. In combinatorial game theory, a certain position where one player can make a threat to win everything but the other can shut that down right now has reason to be considered the smallest positive thing. In the relevant context, fractions like $\frac12$ make sense (and are other game positions). For a bit more detail, see my answer here.