how to find how many domain names an individual person owns?
for example if i want to know who owns this domain name then i can do a whois search and find it if it is not protected by private registrations. but if i want to find the number of domain names and its details that a particular person owns then from where could i fin that information.

Do an alexa search and see the left side of the page. It shows some of the domain names owned by the same person. This is the easiest way to know some of the domain names owned by that person. If you make a list and then continue to check the alexa for each domain name then maybe you will find other domains related to that person.

I have another trick too.

The above trick is good and to extend it.

Do a whois check. List the name and address of the owner into the notepad.

Then do a reverse I.P. check and list the domains hosted on that particular server.

Check each domain's whois and you will find some more as sometimes multiple domains are hosted on the same server by the owner.