Finding Significant Probability from Data I've been recording the results of nearly 2000 card packs being opened. When you open a pack you are guaranteed a rare card but sometimes you get a mythic card instead. I would like to find out what the probability is of getting a mythic card from a pack. From my data I have recorded 220 mythics out of 1874 packs or approximately 11.74%. How do I know if I have enough data points to conclude this is reasonably correct or how do I know if I need more data?

As a simple approach to get a reasonable estimate of the sample size needed you could use Cochran's formula: $$N=\frac{Z^2\,p\,(1-p)}{\epsilon^2}$$ where $\epsilon$ is the desired level of precision (in your case, 0.05), $Z$ is the Z-score that corresponds to the desired confidence interval (in your case, with a 95% confidence interval, $Z=1.96$), $p$ is an estimate of the proportion of the property being observed (mythic card, with estimated proportion 0.1174). You will see that, by doing this calculation, you get approximately $N=159$, well below the size you already have.