Each obolus is comprised of one soul, no more or less. The value of the obolus is inherent in the soul itself. An obolus is a valuable unit of currency, and wraiths will often halve or quarter the coin for trade purposes - oboli will reassemble if left alone, and most wraiths prefer not to think about the division and reassembly of souls.
As with other soulsteel artifacts, oboli have a tendency to mutter and moan, making them rather disquieting. Given the peculiar metaphysics of the Underworld, it is not possible to meaningfully forge an obolus. Any licensed artificer can make oboli out of souls; they pay a 20% tithe to the Stygian government for the right to do so and access to the necessary molds.