14 C.F.R. § 398.6 – Seat guarantees.

(a) The number of seats guaranteed at the eligible place will be sufficient to accommodate the estimated passenger traffic at an average load factor of 60 percent, except that an average load factor of 50 percent will be used when service is provided with aircraft having fewer than 15 passenger seats.
(b) Only under unusual circumstances will an eligible place's essential air service level be set at a number of flights that will accommodate more than 40 passengers a day in each direction (a total of 80 inbound and outbound passengers). Generally, 40 passengers can be accommodated by guaranteeing 67 seats a day in each direction (a total of 134 inbound and outbound seats).
(c) The Department may guarantee an eligible place more than 67 seats a day if:
1. (1) The number of stops between or beyond the eligible place and the hub results in available aircraft capacity being shared with passengers at those other places;
2. (2) The distance between the eligible place and the designated hub requires the use of large aircraft;
3. (3) The eligible place has suffered an abrupt and significant reduction in its service that warrants a temporary increase in the maximum guaranteed capacity; or
4. (4) Other unusual circumstances warrant guaranteeing the eligible place more than 67 seats a day.