Is your statistics good enough to solve this?

You have a population of 19 Twitter accounts and want to take a simple random sample based on their tweet rate (in tweets per day). The pop. standard deviation of all tweet rates is 0.63.

So you know N, (sigma), and you want to have 95% confidence (Z=1.96) and a margin of error of 5% (E=0.05).

Knowing all this, what is the most appropriate formula for sample size estimation in this context?

(You are not allowed to sample the entire population, obviously)