According to a response from Zenimax Online Studious to one TESO fan, multiple ESO Subscriptions will be required for all platforms the game is played on. The only exceptions are PC and Mac platforms which can share one account, but not at the same time.
Explaining this decision, the developer said:
In order to play The Elder Scrolls Online on both PC/Mac and console, players will need to have two separate accounts and subscriptions: one for PC/Mac and one for the console of their choice. Players can use both the PC and the Mac game clients on one account, but an account may only be logged in to one machine at a time. However, console megaservers are separated between PlayStation 4 and Xbox One, so players who want to play on both of those systems will need different accounts (and subscriptions) for each console.
This is the move that the most number of players won’t find surprising even though the decision isn’t too convenient for them.
Source: TamrielFoundry