I had a requirenment to split\move a bunch of site colletcions using same content databse into their own respective content databases.
Here are the steps i followed :
1. Create a New Content Database - Goto Central Admin site -> Application managment
-> Content databases. Choose your destination web application from the left top corner.( the one which will carry all the content databases) and Click on "Add a Content database". See the screen below.
2. Now give this database an appropriate name (according to your site colletcion) and hit Ok. The Database server should already be listed, if not then please enter the database server name too.
3. Now, on the "Manage Content Databases" screen click on the previously used content database. I mean the one with "Site Level Warning" and "Maximum Number of Sites" set to 9000 and 15000. (In most cases).
4. Adjust Site Level Warning and Maximum Number of Sites - Once you open the previously used content database, Set the "Site Level Warning" to the current number of sites in the content database and the "Maximum Number of Sites" to one Plus the current sites in this content database. For e.g. in the screen below I had "2" "Current number of Sites" in my old database so I will adjust
Site Level Warning - 2 and Maximum Number of Sites - 3.