I wrote it out several posts above. Go up and look at it.
You only get Proficiency modifer uses of Favored For and it doesn't bounce on victim death.
You only use it for 1 attack you don't need it to bounce, if you get a second attack out of it great, but it is really just extra damage on one attack:
Attack -hit- Favored foe
Extra Attack -hit-drop concentration-favored foe kill that guy-drop concentration again
Bonus: TWF attack - hit-favored foe.
That is 3 uses in a round against 2 different enemies and it is probably the only time you will use it all day. At high level it is more damage than HM too.
If the fight is hard enough you need more than one attack you are going to use a better concentration spell (and a better spell than Hunter's Mark too).
That's not enough for a 6 encounter day. Not even a 4 encounter day.
It is WAY, WAY, WAY more than enough for a 7 encounter day. I have played multiple Rangers to level 20 and I don't think I ever ran out of Favored Foe past level 5. Not once. I did on occasion run out of spell slots, but I did not do that often either.
If you insist on attacking every turn and not casting other spells you might run out of FF, but most of the time at level I am casting spells or supporting concentration on something better (like Spike Growth, Web, Summon Fey, CWB or Conjure animals) when I am attacking with a weapon and using a spell to support that attack, I am often using a different and better concentration spell (like Ensnaring Strike or Zephyr Strike or Guardian of Nature).
Favored Foe is for when you don't need any of that. When that happens you spam it like Paladin Smites because you are not concentrating and you want to spend it when you get the chance.
Take a 13th level Fey Wanderer for example on a
7 encounter day:
1. I can cast Summon Fey 4 times a day at 3rd level and I am probably doubling some of those up -
so lets say that is covering 3 fights total
2. I usually have Cause Fear on a free cast from Shadow Touched and probably cast it using a 2nd level slot once a day as well. Let's assume I lose concentration so we will call that only 1 fight for two castings (one free one 2nd level) since I have to recast it -
that is 4 fights total
3. I have two more 2nd level slots. Let's say that is one fight I cast silence and one fight I cast spike growth -
that is 6 fights total
So now I have 1 more fight left, I was using concentration on something else for all 6 other fights. I still have four 1st level slots left, a 4th level slot, a free cast of Speak With Animals, Beast Sense, Speak With Plants, Locate Creature, Invisibility, and four uses of favored foe.
That is WAY, WAY, WAY more resources than I can use up in one fight and among those resources the 1st level slots and probably the Invisibility are all better uses of your concentration than Favored Foe (or Hunter's Mark) would be.