Warning: contains major spoilers for Avengers Mech Strike #4!
Thanos is allying himself with the Avengers in order to beat Kang, which means providing them with some of his vast resources, including his army of followers. No matter the mission, Thanos has rarely struggled to find minions to support him, and Avengers Mech Strike #4 explains where his seemingly endless regiments of cannon fodder come from.
Introduced in The Invincible Iron Man #55 in 1973, Thanos has since become one of the Avengers' worst enemies. He has already made his MCU debut, portrayed by Josh Brolin, and remains the biggest foe that Marvel's movie superheroes have faced thus far. He is best known for using the Infinity Gauntlet to wipe out half of all life, but this time, he doesn't have any all-power artifacts - just a huge army willing to follow his every command.
Avengers Mech Strike #4 - from Jed MacKay, Carlos Magno, Guru-eFX, and VC's Cory Petit - features Thanos and his army as they're temporally displaced by Kang the Conqueror. Thanos agrees to work with the Avengers, finding it amusing that while he has yet to meet them in his own timeline, the assembled heroes clearly hate him. Planning their attack, Black Widow suggests a fake assault to draw Kang's attention, and Thanos casually offers to commit his "legions of space-pirate scum." When Spider-Man questions Thanos' willingness to sacrifice his people, the Mad Titan observes that his current servants aren't special, and that "the galaxy is filled with bloodthirsty rabble, just waiting for a powerful leader for whom to throw their lives away." It's clear that rather than deliberately assembling his army around a shared cause, Thanos attracts followers simply due to his power and influence, welcoming all those who wish to scavenge in the wake of his various conquests.
Spider-Man and the Avengers disagree with Thanos' method of sacrificing his army, but Thanos truly doesn't care. He reveals that his army doesn't mean anything to him. He is a powerful leader that beings will flock to, and he is ready to sacrifice them for his cause, confident in the knowledge more will fill their place. The Mad Titan's army is so large because he exploits a never-ending resource - people looking to piggyback on existing power without caring how it's used. Thanos' army is also far bigger than many would guess. Thanos vol. 2 #1 revealed that Thanos stables some of his forces in the Black Quadrant, a grouping of planets and moons used purely to house his armies. In the context of a following spread across multiple planets, it makes sense why Thanos would be so unconcerned about committing troops to an attack on Kang's base on Earth.
Thanos is a very powerful being, so it makes sense that he would easily gain a following among those who simply want the martial strength to live well. While some leaders actually care about their armies and rally them in effective ways - Doctor Doom at least professes to love his people, while Magneto is a bona fide freedom fighter to the mutants who make up his Brotherhood - Thanos has no such respect for his troops, yet thanks to the galactic scale on which he operates, his ranks still dwarf those of other villains. The Mad Titan seems to keep a very hands-off approach, while readily sending his soldiers straight to their deaths with no remorse. For less powerful beings, this method wouldn't work, but for a force of nature like Thanos, it doesn't hinder his ability to amass soldiers at all. Thanos' forces are never-ending because he appeals to the worst part of human (and alien) nature, meaning he'll never run out of people willing to do his dirty work.