Does every Site Collection in SharePoint (2010) have its own Content Database (by default)? need some information, thanks.
Not by default. If you go to the Content Databases section of Central Administration you'll see two properties associated with a content database. One is an alert level that will notify you when that many sites are now on that content database, the other is a physical maximum number of sites that's allowed on that content database.
It is possible to create a content database and set its' limit to 1 so it is dedicated to a site collection, but out of the box, the default values are quite high.
Note that if you need to gather quick information about how many sites are on which database, you can use the Get-SPContentDatabase cmdlet in PowerShell as
SharePoint will not create additional content database unless you request it though Central Administration or PowerShell
When you create a Site Collection though the UI SharePoint will put it into one of the Content databases trying to even out the load. If you create a site collection through PowerShell (or code) you have the option of specifying the content database to use
You can move a Site Collection to another content database using PowerShell if you want