There is no concrete answer, but let's try do some estimation.
According to guildstats.eu/bosses, 21914 Ferumbras Mortal Shells have been killed since the implementation of this boss to the game.
Taking into consideration the fact that scroll is being sold so rarely or that seeing a player using Rift Runner mount is an unique sight even now... If the scroll is the rarest loot, let's assume that the chance to loot is 0.01%.
So it would mean that approx. 22 scrolls have been looted up to this day and in my opinion that number sounds reasonable.
Note that these are just my assumptions confronted with some stats&logic.