Before moving sites between content databases, you should know a quantity of your databases for your WebApplication. You can move sites between content databases only if they are in the same WebApplication!

To see information about all your content databases with powershell, you need to enter to PowerShell Console:

The result is something like this:

Item "CurrentSiteCount" shows you a quantity of site collections in the content database. It’s not easy to say which value is the best – it depends on size, update rates, users etc. But if it’s your SharePoint farm, you should know better about site collections.