If the forum is just on the threshold, one admin along with a moderator are enough. Though, there are some aspects which need to be taken care of to maintain the quality of the posts, still too many staff at the beginning might spoil the cause.
However, if the forum becomes popular, and the number of users increases in the course of time, the count of staff members may require to be increased. The standard should not get compromised due to the lack of support staff.