Basis for fees? (is it number of email addresses)

I am trying to use Office 365 for 6 People that work for my company. $48 per month is my expected fees. However, I have 30 - 40 other email addresses for our partners and independant referers of business, They have no need for Office 365 functions other then email. If I implement O 365 for the 6 of us, what are the charges if I have 30 other email addresses? Thanks I appreciate you help