Answer
$2000$ fans
Work Step by Step
$z=\frac{y-\mu}{\rho}$
$y=\mu+z\times\rho$
$\mu=21,359~fans$ and if $z=\pm2$ :
if $\rho=20$ then $y=21,359\pm2\times20$ then
$21,319\leq y\leq21,399$
if $\rho=200$ then $y=21,359\pm2\times200$ then
$20,959\leq y\leq21,759$
if $\rho=2000$ then $y=21,359\pm2\times2000$ then
$17,359\leq y\leq25,359$
if $\rho=20,000$ then $y=21,359\pm2\times20,000$ then
$-18,641\leq y\leq61,359$
It seems that a standard deviation of $2000$ fans is most likely.
