What is the max amount of achievements I can get by performing any one action?

I think I've found another one:

You are a Spy disguised as an enemy Spy backstabbing that same Spy who is

• On your friends list
• Has his revolver out
• Is dominating 3 of your teammates
• Is also disguised as a Spy
• Is equipping the Balloonicorn and is your 79th Balloonicorn victim

You get Revenge on that Spy and get your 1000th stab on top of a Control Point that just became available because the other spy captured it while being healed by a medic and you being covered in Jarate and after having that enemy spy trigger your Dead Ringer and you exiting a teleporter and having 4 prior spy achievements, and this kill is your 5th kill in a row without dying, gives you:

• Come in from the Cold (revenge)
• High Value Target (stab someone with 3 dominations)
• Spymaster (1000 stabs)
• Agent Provacateur (stab 10 friends)
• Identity Theft (stab the guy you are disguised as)
• A Cut Above (kill a revolver-wielding spy with your knife)
• Insurance Fraud (get a backstab while an enemy medic heals you)
• Sleeps Agent (kill the guy that triggered your DR within 20 seconds)
• On Her Majesty's Secret Surface (start capping a point as soon as it becomes available)
• Counter Espionage (backstab a disguised spy)
• Wetwork (kill someone while jarated)
• Point Breaker
• Milestone 1
• Milestone 2
• Milestone 3
• Costume Contest (kill a spy disguised as your current class)
• Nemesis (5 revenge kills)
• Riftwalker (get a kill shortly after exiting a tele)
• Hard to kill (5 kills in a row without dying)
• Hardcore (get 1000 total kills)
• The great deflate (kill 79 people with balloonicorns)

Total achievements from 1 stab: 21 achievements

Apparently 25 is possible as Sniper, courtesy of Reddit:

Setup

You must have exactly:

• 4 Sniper achievements
• 999 kills (all Sniper)
• 1 prior Domination
• 78 Balloonicorn kills
• 24 parachute headshots
• 9 Sniper headshots
• 4 prior Revenge kills

Equipment

• Pyrovision goggles
• The Machina
• The Razorback

Action time!

1. Take a teleporter
2. Take a fully-charged shot
3. Your fully charged shot must kill all of the following:
   • A sniper,
     • Whom you've killed 3 times
     • Who is wearing
       • Pyrovision
       • A Balloonicorn
   • A Spy,
     • Who recently tried to backstab you
     • who is now cloaked
   • A second Spy who just uncloaked
   • A Demoman
     • Who is parachuting with the Base Jumper, following a sticky jump
     • With a headshot
   • A second Demoman
     • Who is charging towards you
     • Who is also dominating you
   • A Medic
     • With full Uber
   • A Scout
     • During a double-jump
   • A sentry gun
4. Taunt (well, who wouldn't after all that?)

Sniper-specific Achievements

• Australian Rules
• Be Efficient
• De-sentry-lized
• Dropped Dead
• Flight Cancelled
• Jumper Stumper
• Kill Everyone You Meet
• Not a Crazed Gunman, Dad
• Parting Shot
• Self-destruct Sequence
• Shock Treatment
• Shoot the Breeze
• Socket to Him
• The Last Wave
• Triple Prey
• Überectomy
• Sniper Milestone 1

General achievements

• Nemesis
• Riftwalker
• The Great Deflate
• Hardcore
• Hard to Kill
• Emergency Brake
• BFF²
• A Fresh Pair of Eyes

Total: 25 achievements