Can I use one account, or do I need a separate Firefox account for each profile?

Hi

What would be the purpose for using a Firefox account for syncing ? Do you want to keep data from some profiles separate like work and home ?

When you connect devices to the same Sync account then data from each connected device will be uploaded to the Sync server and merged to the other devices (i.e. there is no main master device). If you would connect multiple profiles on a device or other devices to the same Sync account then all these profiles will end up with the same data that is part of syncing data.

To keep data separate you either need to exclude this data from syncing or you need to use separate Sync accounts with their own email address.