How does multi-store support work?

By default, if you don't make any configuration changes, then all customers will be imported into your MageMail account from all of your stores.

If you want your MageMail account to be associated with just a specific set of stores, there are two options to set that up:

Option 1 - Account-wide

Go into Settings>Filters and you'll see the list of stores:

After selecting your stores, one thing I like to always do is go into the Subscribers tab and spot check that the number of subscribers it's showing is lower than it was before you enabled that filter:

Option 2 - Message-specific

Don't enable any account-wide store filter, but instead you can segment your individual messages by store.

This way you could send one message to your English/French customers, and another message to German customers, without requiring two separate accounts.

Currently, each MageMail account can only have one store url though which is used for things like product url's.