No, you do not need more than one steward account. One steward account can edit and maintain multiple Little Free Libraries in the mobile app. When you finish setting up your steward account, each of your Little Free Libraries (identified by their unique charter number) will be assigned to your steward account.
You can edit each of your libraries or transfer one or more of them to another person from the My Libraries section of the mobile app or the steward web portal.
Once you set up your steward account, you can transfer the appropriate libraries to the actual stewards. When you transfer a library, you simply trigger an email to the potential new steward. The email has a link for that person to set up a steward account and “claim” the transferred library. When the new steward completes their account setup, the library is automatically reassigned to the new steward.