On first launch, WordPress prompts you to input your MySQL database username, password etc. Then I can start working from dashboard.
Is it possible for WordPress to have more than one database?
If yes, what is the possible example for having 2-3 databases? When would you need more than one db?
Or the working rule is: one blog - one db? If you have 2-3 blogs, can you manage them from the SAME dashboard!? And each must of course have 1 db?
Yes you can have more than one database, this type of set-up is for large traffic heavy sites and you would want to use something like http://wordpress.org/plugins/hyperdb/ . This is outside the scope of the vast majority of WordPress sites.
You can manage a multitude of blogs from 1 db if you want using http://codex.wordpress.org/Create_A_Network. The option is up to you, the bare minimum is 1 db per install.
or the working rule is: one blog - one db?
Kind of... It would be one blog - one set of db tables. When you install WordPress, you will give the required db tables a prefix. Using a second prefix would allow you to have a second WP installation running in the same database. And a third, fourth, and so on.
if you have 2-3 blogs, you can manage them from SAME dashboard!? and each must of course have 1 db?
In a basic install of WP, no. While you can have multiple WP blogs in a single db, you can only manage one blog from one dashboard.
That said, you could use WordPress multisite, but that's more involved that a standard/basic install. Also, there are some third party products that allow you to manage multiple installs through one dashboard, but that's not WP by itself. Both of these options would still only require one database.
