I'm finding visual studio 2005 a very interesting development tool.

When using asp.net website administration tool for the first time  in visual web developer , a database  named ASPNETDB.MDF is automatically created in the App_Data folder, my question is "Is it a good practice to add table into this database that was generated or a new   database will be created each time that will be added to the app_data folder now making the database base n the App_Data folder more than one"

You can add tables to the aspnetdb that is automatically created.Whether you want to do so or not is your call. If you intend to share the membership database across different sites then its advisable to keep the aspnetdb separate and add your  application specific tables to a different database.