I remembered this really neat internal function that is handy to display anywhere you want to show how many sites and users you have in your network.
$stats = get_sitestats();
echo 'There are currently '.$stats[ 'blogs' ].' sites running on this network and '.$stats[ 'users' ].' users.';
Surround that in php tags.
For added snazziness and extra bonus points on your homework, go grab a copy of the blank widget Blank Build-A-Widget (992) paste this code in there, and you’ve just made a network stats widget. Awesome, you rock