I believe on most aircraft & flights within the USA, FAA staffing minimums are 1 crew (FA) to every 50 passengers. I'm not 100% sure on this; perhaps someone knows the exact language with respect to how the FAA determines crew minimums.

If on a 737 with 129 seats, I would expect 3 Flight Attendants. It's possible there may be an extra one, such as an FA in refresher training that just got his/her job back, but considering the financial conditions many US domestic carriers are in, they often go with the minimum.

With respect to larger aircraft such as the 747 or 777, it's possible a different set or varation of rules apply based on the number of exit doors and passengers. Not many US domestic carriers operate these on US domestic routes; I believe UA has a couple domestic 747s in a three class config, while DL, CO, AA and UA each have a few domestic 777 flights as well on trunk routes to feed international traffic.

Additionally, staffing may be higher than the minimum on some flights because of premium cabins, crew rest periods, etc -- but we're primarily talking about long-haul international here. On a recent 8 hour flight between Melbourne & Kuala Lumpur we had 2 Capitans and one First Office aboard because of the 8 hr duration, the FA to passenger ratio was very good up front, and I'm sure there was extra service crew on-board to handle breaks. 8 hrs while long is really more of a medium-long haul flight; but for a 777 on short a DEN-LAX flight, as an example, I wouldn't expect extra crew, whereas I would for DXB-EWR which came in around 14 hours (scheduled 14 1/2), despite both being 777 operated. The 8 hour MEL-KUL was also a 777, FWIW.

Anyways, with respect to most planes, I think if you calculate 1 FA to 50 passenger seats you're probably good, unless it's a 3 cabin domestic with premium service then there might be a higher ratio. Otherwise, 189 seats = 4 crew and instead of asking, perhaps provide 4 gifts. Just a thought at least!

Best,

SDF_Traveler