Content databases per site collection:
How many content databases can I have per site collection?
How many site collections per content database?
Many. According SharePoint 2010 boundaries and limits, you should limit “the number of site collections in a content database to 2,000. However, up to 5,000 site collections in a database are supported.” There is a recommended 200 GB maximum for the size of each content database. If you approach that limit, you can break out individual site collections into their own content databases.