I am looking for the average number of political magazine
subscriptions that U.S. households who subscribe to at least one
political magazine have.

Examples of political magazines on the right, include:  The National
Review, The Weekly Standard and Human Events; examples on the left,
include: The Nation, Mother Jones and The American Prospect.

For instance, I am looking to complete the following statement:

The households in the U.S. that subscribe to at least one political
magazine have an average of X subscriptions to political magazines.