Bag End Porter 

Creature — Dwarf
Whenever this creature attacks, it gets +X/+X until end of turn, where X is the number of legendary creatures you control.
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Cards that do something similar expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Life of the Party 97%
$11.75#5019
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the number of creatures you control."
Akroan Hoplite 96%
$0.07#15923
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the number of attacking creatures you control."
Orcish Siegemaster 90%
$0.37#4737
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the greatest power among creatures you control."
Rubblebelt Rioters 90%
$0.17#17740
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the greatest power among creatures you control."
Alpine Houndmaster 90%
$0.16#22386
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the number of other attacking creatures."
Consul's Lieutenant 87%
$0.18#21483
"Whenever this creature attacks, if it's renowned, other attacking creatures you control get +1/+1 until end of turn."
Impetuous Protege 87%
$0.19#27019
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the greatest power among tapped creatures your opponents control."
Team Avatar 87%
$0.15#10415
"Whenever a creature you control attacks alone, it gets +X/+X until end of turn, where X is the number of creatures you control."
Angelic Exaltation 87%
$0.32#9811
"Whenever a creature you control attacks alone, it gets +X/+X until end of turn, where X is the number of creatures you control."
Vile Deacon 85%
$0.18#25199
"Whenever this creature attacks, it gets +X/+X until end of turn, where X is the number of Clerics on the battlefield."
Charging Hooligan 85%
$0.14#27011
"Whenever this creature attacks, it gets +1/+0 until end of turn for each attacking creature. If a Rat is attacking, this creature gains trample until end of turn."
Dauntless Veteran 84%
$0.13#16001
"Whenever this creature attacks, creatures you control get +1/+1 until end of turn."
Inspiring Unicorn 84%
$0.18#15870
"Whenever this creature attacks, creatures you control get +1/+1 until end of turn."
Wandering Musicians 84%
$0.08#20535
"Whenever this creature attacks, creatures you control get +1/+0 until end of turn."
Shared Animosity 84%
$3.76#672
"Whenever a creature you control attacks, it gets +1/+0 until end of turn for each other attacking creature that shares a creature type with it."
The Long Reach of Night // Animus of Night's Reach 84%
$0.22#11099
"Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the number of creature cards in defending player's graveyard."
Rampaging Classmate 84%
$0.27#25040
"Whenever this creature attacks, it gets +1/+0 until end of turn for each other attacking creature."
Golden-Tail Trainer 84%
$0.24#9820
"Whenever this creature attacks, other modified creatures you control get +X/+X until end of turn, where X is this creature's power."
Aurochs 84%
#29850
"Whenever this creature attacks, it gets +1/+0 until end of turn for each other attacking this card."