A single jury is still too small of a sample size to be statistically significant. If you have more than two groups in the population (races in this case) then you need at least 5 expected cases of each in order for analysis of the actual versus expected proportions to be meaningful, which you can't get with 12 jurors. The closest you can get to it is if there are 3 groups evenly divided in the population, which gives you 4 expected cases each. If you had only 2 groups in the population that were each 50% then you would have 6 expected cases each. However, when analyzing sample proportions of 2 groups you need at least 10 cases each.
Those minimum case numbers are needed in order for the central limit theorem to be applicable in the statistical analysis.
Huh, interesting, thanks for the explanation. Is there a nice summary/term I can search for that explains your needed sample side for things to be statistically significant?
Comparing proportions is the general topic. You are typically comparing a sample proportion (like the demographics of a jury) to a population proportion (like the demographics of a community from which jurors are drawn).
Those minimum case numbers are needed in order for the central limit theorem to be applicable in the statistical analysis.