Its one dose because its probably concentrated antiserum from someone who survived Ebola. There may be only a few survivors, who could be suitable donors and do not have other infections like malaria, HIV, hepatitis, TB and other co-morbidities.
The more daring companies and organizations are likely testing their stuff as a last ditch effort. If it works great, if not, they modify the drug and try again. NNVC is unlikely to get to test the ebolacide again for a long time.