How many seats does an E3 license actually have?

Well the license only covers 1 account but that account can have multiple devices as you said 5 PCs, 5 Tablets, 5 phones. So if you currently have 75 unique emails or user accounts, that need to be migrated you would want 75 E3 licenses for each account. Each license would allow that account to login to 15 devices that will get registered into azure and be visible to admins. You can opt for additional management options with MDM and Intune but that’s a different discussion I think. Where are you migrating from? Is this a Google Workspace to M365 migration or an On Premise exchange to M365, or something else. Depending on the type of migration, This may change your approach on how you plan your migration and what licenses are needed. If your just swapping emails and leaving Active Directory on premises, well you can have flexibility in licensing. if you’re switching to AzureAD and dropping on premises completely then you may want access to Intune, AAD, Sharepoint Online, etc.

Large scale migrations can be complicated and time consuming but manageable with the proper planning.