“When The Guild attacks a region that has a Stellan it also steals 1 Corra.”
Does having multiple Stellen in a region allow you to steal multiple Corra? (no)
This ability works the same whether there is a single Stellen or multiple Stellen. Having more than one Stellen in the region does not allow you to steal multiple Corra with a single attack, it is still just 1 Corra if the region has at least one Stellen in it.
Who does the Guild steal the Corra from, and does that faction need to have Corra in order for it to be stolen? (the faction they attack, and yes)
The Guild steal the 1 Corra from the faction they are attacking. They can only steal Corra if the attacked faction has Corra to steal - they do not take a Corra from the supply if the attacked faction has no Corra, instead they just do not gain Corra.