One way to answer your question is to answer a similar question: what is the probability that any given number in A was not picked after $N$ iterations?
The answer to this question is: $P=(1-\frac{30}{1000})^N$
Then the answer to your original question is $\frac{\ln (1-p)}{\ln(1-\frac{30}{1000})}$, where $p=!-P$$p=1-P$ is your certain probability.
Also, a word certain usually refers to probability 100% or 1. I'm assuming that you use this word in its different meaning, i.e. "specific but not explicitly named or stated".