During Army of the Dead, Zeus is building an army of Alpha zombies, and viewers see quite a few of them in action on screen, but in the six years since Vegas was walled off, how big has he grown his army? When Army of the Dead opens and Zeus (Richard Cetrone) escapes, he turns at least two of the soldiers into Alpha zombies and then attacks Las Vegas. The opening credits sequence mostly depicts shamblers overrunning the city, so it's not clear how many Alphas he created during that time, but after Vegas is walled off, he spends 6 years biting and turning more people, especially refugees from the camp who sneak into the city.
Zeus' ultimate goal is to create an Alpha zombie baby, making way for natural procreation between Alphas so they don't need to use violence to spread anymore, but he hadn't accomplished that goal yet, so every single Alpha zombie seen in Army of the Dead was bitten by Zeus (except for the robot zombies, presumably). There's quite a few seen in the pool scenes, but as they pour out of the Olympus hotel for their final assault, it's clear there's even more. Fortunately, director Zack Snyder has given a number for how many Alphas there are.
In the book Army of the Dead: A Film by Zack Snyder: The Making of the Film by Peter Aperlo, Snyder is quoted saying "The army at its core is maybe five hundred Alphas. Over the years, Zeus has been slowly turning people into Alphas who have wandered into Vegas or have tried to, mostly from the refugee camp." The crew in Army of the Dead is able to cut down quite a few of them, although none of them make it out unscathed other than Kate (Ella Purnell), while Vanderohe lived, but has a bite at the end. It's easy to imagine how under other different circumstances, an army of five hundred Alphas could pose quite a threat, especially if they're able to wear armor and use weapons like Zeus.
With that many Alpha zombies, it's impressive they didn't get out of the city and spread even farther, but it should be noted Zeus only ever attacks defensively or in retaliation, so the five hundred zombies may have been more for defense than any sort of grand attack planned by Zeus. In fact, since if Zeus had been successful in creating an Alpha zombie baby so the Alphas could procreate without biting humans anymore, it's possible the Alpha population would have eventually increased enough to pose a different kind of problem to the United States.
With such a large force, the fact Zeus kept the Alpha zombies secret is a fascinating aspect of the nature of the Alpha zombies. Since they're the product of some kind of experiment involving aliens, there's still a lot of questions around what the Alphas actually are and what their true purpose is. With the Army of the Dead questions and more could be coming.
