Let's first remove the confusion around the metapackage.
A meta-package is just an empty package which pulls other packages, but when removed, doesn't cause those pulled packages to get removed. The whole purpose of existence of these packages is to help get a selected set of packages. It helps greatly to install a complete Desktop Environment which provides a complete set of desktop applications.
See the question here What is the difference between a meta-package and a package?
You can easily see them in Synaptic -> Left-Panel -> Click Sections and Select Metapackages.
As for other types of packages, every single package that needs another package is a top level package. So, by definition there are thousands of them. Only the most basic packages are not top level package.
Hope that helps.