yes, one stack per drone ... and the mod tries to keep the depot filled at least with this amount, thus it can also be a bit more. since that is also multiplied by the researched capacity bonus, you may end up with 6-12 stacks per drone you put in the depot. this problem is currently discussed in several other threads ...
until then be careful to not research too much capacity bonus too early and use more drones in the depot instead (6 drones without bonus result in needing more fuel, but delivering only 6-7 stacks; and you always can use fewer drones, but currently not less bonus)
