The phrase "countless zombies" means there are too many zombies to be counted or too many to keep track of because they are in large numbers. Full definition
Being a survival horror game, you come up against countless zombies. (nextgengamingblog.com)
Help Hank fend of countless zombies in Dead Rising 4 on Xbox One, which is available right now for # 24.85. (jelly.deals)
When asked what he believes separates Days Gone from the other countless zombie - filled games of the past decade, Garvin replied, «We are going to be, I think, one of the first open world games that's dangerous all the time.» (mammothgamers.com)