Are air marshals on every flight?
The exact number of air marshals is not public information, and they certainly don't disclose what flights they fly on or other operational details. However, the number has been reported to be on the order of ~3,000-4,000 and reports indicate the number has decreased somewhat since then. All of them are, of course, not on duty 24/7/365. There are around 87,000 flights a day in the US alone, but only around 28,000 are commercial passenger flights, where air marshals are presumably most likely (hopefully there's not a need for air marshals on military flights, for instance).
So for some back of the envelope math, let's conservatively say there are 3,500 air marshals who all work five days a week, they all work alone (in reality, they appear to work at least in pairs), and they somehow manage to average protecting four flights a day. Even with these conservative assumptions, that would only be 5000 flights/day covered by air marshals. From this, it should be clear that there cannot possibly be an air marshal on every commercial flight in the US.
The President of the Air Marshal Association/CWA (their union), not entirely a disinterested party, agrees:

&quot;There are around 30,000 commercial flights per day over the U.S.,&quot; says Casaretti. &quot;If you were to attempt to place a team of just two FAMs on each flight, it would require an agency of over 75,000 FAMs (accounting for training and days off). FAMs cover a very small percentage of commercial flights.&quot;
Which flight air marshals get on is determined by a computer program that assesses the probability of threat based on the aircraft, departure and destination cities, as well as the amount of fuel on board. This is the threat matrix that comes out of the missions operation center.
The TSA will not release statistics on the number of air marshals. TSA's Pascarella acknowledges only &quot;many thousands.&quot; But Biles estimates that there are approximately 3,300 FAMs and of those, 34% are filling ground assignments in training, operations and management.
&quot;We call them 'chair marshals,' riding out their career in management,&quot; says Biles. That leaves 66% of the workforce to perform in-flight security duties.
&quot;If one accounts for vacations, sick leave, medical leave and days off, some air marshals working in operations have told me that this accounts for less than one half percent of all U.S.-flagged aircraft being covered by federal air marshals,&quot; he says.