No, I don't think he's got a stash of riders.. he's WAY too paranoid for that... but I don't doubt that he's got a cache of 'hearts' tucked away... just a question of whether they're right near him (likely as at least with Eragon he's got to have physical contact with a big freaking stone to 'feel' Glaedr) or whether he's tucked away a cache somewhere other than his person that Varden, etc could get to... if they could find it, bypass enchantments, etc.
All that leads me to think the Vault of Souls is a vault of Dragon souls... that perhaps Eragon can use to defeat him... then he can take those, as well as Galby's in the ship (why else would two riders NEED a boat unless to carry a boatload of cargo) off to wherever it is they end up going... I'm leaning to someone... Arya, Glaedr (ala Oromis's memories) or another elf fiinally figuring where they've heard that before and directing Eragon to find it... thus he'll be as powerful/able to defeat Galby... but who knows...