Office 365 Enterprise E 3 offers include unlimited number of users and since you are signing up for a trail to develop into a paid plan. Making use of 25 users in the trial will suffice.
Office 365 Business can accommodate a maximum of 300 users only.
References:
https://technet.microsoft.com/en-us/office/dn788955
https://technet.microsoft.com/en-us/library/office-365-plan-options.aspx https://technet.microsoft.com/en-us/library/office-365-platform-service-description.aspx
Katzer, Matthew and DonCrawford, Office 365 Migrating and Managing your Business in the Cloud, Apress Media, New York, 2013, pp 84-87
