What do you consider a ''Test Plan''? Test Plan is the name of a module which contains Test Folders and Test Cases.
You could write SQL queries to count the number of appropriate entities in each project's database, and tally up the counts. I wrote a console application in Visual Basic that actually reads the Site Admin schema to get a list of the projects and their database names, then executes SQL queries against each database to count the number of many of the different entities (i.e. Requirements, Tests, Test Steps, Test Instances, etc.). I run this program weekly to help me monitor which projects have activity and how the projects grow or shrink over time.