How can I know how many websites are hosted?

probably a good idea...you might want to get one that comes with a dedicated ip

You can tell how many sites are on a shared server by using whois.sc

i.e.: http://whois.sc\ozzo.com

It will also show you six site URL's with the option to see them all with a paid subscription.

Actually 19 sites on a shared server is very low, many companies will host 200-300 sites per server (obviously oversold).