is it possible to license only specific users

Hi All!

I have a mail server with 9 domains approx 300 mailboxes. Only one of them is the corporate domain with 17 mailboxes, all the others are remote offices. I would like to activate mobile for just the corporate domain and users. Can I purchase 20 mobile licenses and apply them only to corporate, or do I have to purchase one for every mailbox that is on the server?

Thanks!