Question: how many Infinity Gems must earths heroes seek out? 

Context articles: Infinity Gems
Batmen of All Nations
Disney Infinity: Marvel Super Heroes
Marvel Super Heroes In War of the Gems
Earth-Two
Infinity
Superman and Batman: World's Funnest
GURPS Infinite Worlds
Earth-One
Infinity Watch

The Infinity Gems, originally referred to as Soul Gems, are six immensely powerful fictional gems appearing in American comic books published by Marvel Comics.
 Whoever holds all six gems and uses them in concert, such as in the Infinity Gauntlet, gains virtual omnipotence and omniscience.
 The six primary gems are the Mind Gem, the Soul Gem, the Space Gem, the Power Gem, the Time Gem and the Reality Gem, and in later storylines, crossovers and other media, a seventh Infinity Gem is sometimes added.
 The Gems play a prominent role in the first three phases of the Marvel Cinematic Universe, where they are known as the Infinity Stones.
Batmen of All Nations are a group of fictional superheroes appearing in American comic books published by DC Comics.
 The characters were inspired by the superhero Batman to fight crime in their respective countries.
 The group first appeared in Detective Comics #215.
 Later the Batmen of All Nations were renamed the International Club of Heroes, often known as just the Club of Heroes.
 Post-"Crisis on Infinite Earths", they were named the Dome (see Global Guardians) and were not inspired by Batman but the Justice Society of America.
Disney Infinity: Marvel Super Heroes (also known as Disney Infinity 2.0 Edition) is an action-adventure sandbox toys-to-life video game developed by Avalanche Software and published by Disney Interactive Studios.
 The game features additional work by Studio Gobo and Ninja Theory.
 It is the sequel and successor of the 2013 video game "Disney Infinity" and was announced on April 8, 2014.
 The game was released on September 23, 2014 in North America, September 19, 2014 in the United Kingdom, September 18, 2014 in Australia and the rest of Europe for iOS, PlayStation 3, PlayStation 4, Wii U, Microsoft Windows, Xbox 360 and Xbox One, and for PlayStation Vita on May 9, 2015.
 This game was also released around the fifth anniversary of Disney's acquisition of Marvel.
Marvel Super Heroes In War of the Gems (マーヴルスーパーヒーローズ ウォーオブザジェム , Māvuru Supā Hīrōzu: Wō obu za Jemu ) is a 2D action platformer video game developed by Capcom for the Super Nintendo Entertainment System, based on the events of Marvel Comics' series, "Infinity Gauntlet".
 In the game's plot, Adam Warlock calls upon Earth's greatest superheroes to seek out the Infinity Gems before they fall into the wrong hands.
Earth-Two is a fictional universe appearing in American comic book published by DC Comics.
 First appearing in "The Flash" #123 (1961), Earth-Two was created to explain how Silver-Age (Earth-One) versions of characters such as the Flash could appear in stories with their Golden Age counterparts.
 This Earth-Two continuity includes DC Golden Age heroes, including the Justice Society of America, whose careers began at the dawn of World War II, concurrently with their first appearances in comics.
 Earth-Two, along with the four other surviving Earths of the DC Multiverse, were merged into one in the 1985 miniseries "Crisis on Infinite Earths".
 However, following the events of Infinite Crisis, the Multiverse was reborn, although the subsequent Earth-Two was not the same as its pre-Crisis equivalent.
Infinity (symbol: ∞ ) is an abstract concept describing something "without any bound" or "larger than any number".
 Philosophers have speculated about the nature of the infinite, for example Zeno of Elea, who proposed many paradoxes involving infinity, and Eudoxus of Cnidus, who used the idea of infinitely small quantities in his method of exhaustion.
 Modern mathematics uses the general concept of infinity in the solution of many practical and theoretical problems, such as in calculus and set theory, and the idea also is used in physics and the other sciences.
Superman and Batman: World's Funnest is an American single issue prestige format comic book published in 2000 by DC Comics.
 It was written by Evan Dorkin and illustrated by many artists.
 It is an Elseworlds tale and as such is not considered part of the main DC canon/continuity.
 Despite the title, Batman and Superman play only a small role in the story which stars instead Mr. Mxyzptlk and Bat-Mite as the main protagonists.
 The book pokes fun at many comic book conventions and DC heroes from the golden through to modern ages.
 Its setting is a multiverse similar to the pre-crisis DCU but also includes references to other Elseworlds tales ("Kingdom Come" and ""), the modern DCU, the DCAU and even pays a visit to "Crisis on Infinite Earths".
GURPS Infinite Worlds is a supplement for the Fourth Edition of the "GURPS" role-playing game, published by Steve Jackson Games in 2005 and written by Kenneth Hite, Steve Jackson, and John M. Ford.
 It expands upon the campaign setting of conflict between the "Infinity Patrol", which is the time-travel agency on "our" Earth, referred to as Homeline, and "Centrum" across a multiplicity of alternate history Earths.
 This was presented in the "Fourth Edition GURPS Basic Set" (and originated in the Third Edition supplements "GURPS Time Travel", "GURPS Alternate Earths", and "GURPS Alternate Earths II").
Earth-One (also Earth-1) is a name given to two fictional universes (The Pre-Crisis and Post-Crisis versions of the same universe) that have appeared in American comic book stories published by DC Comics.
 The first Earth-One was given its name in "Justice League of America" #21 (August 1963), after "The Flash" #123 (September 1961) explained how Golden Age (Earth-Two) versions of characters such as the Flash (Jay Garrick) could appear in stories with their Silver Age counterparts (Barry Allen).
 This Earth-One continuity included the DC Silver Age heroes, including the Justice League of America.
 Earth-One, along with the four other surviving Earths of the DC Multiverse, are merged into one in the 1985 miniseries "Crisis on Infinite Earths".
 This Earth's versions of characters were primarily the Earth-One versions (i.e. Superman, Batman), but some characters from the four other worlds were also "folded" in.
 In "Infinite Crisis", Earth-One was resurrected and merged with the primary Earth of the publication era to create a New Earth that brought back more aspects of Earth-One's original history.
 In 2007, a new version of Earth-One was created in the aftermath of events that occurred within the " 52" series.
The Infinity Watch is a group of fictional characters appearing in American comic books published by Marvel Comics.
 The Watch was gathered in "Warlock and the Infinity Watch" #1, and starred in that series until it ended with issue #42.
 The six members were the self-appointed guardians of the Infinity Gems, which were each given to a single member in order to safeguard against anyone else assembling them into the Infinity Gauntlet.