I have often wondered this while visiting ancient historic sites with obvious religious connotations. There is a tendency to view historic societies as homogeneous, with each member having the same beliefs, practices and rituals as their peers. This is obviously not the case as in any society there is a variety of beliefs being practised at any given time.
I know that the concept of atheism as we know it today originated in the west with the Ancient Greeks, but surely some sort of disbelief of gods must of existed before then? What evidence do we have to suggest the existence of such (dis)beliefs? What percentage of people were actively religious? Is there even anyway of knowing?
I can only speak on Indian history. There's a lot of evidence of atheism, or more generally non-theism in historical India. I will focus on one school, though there are a couple of others.
Charvaka:
This is a materialist school of thought that rejected the existence of god(s) and many of the other spiritual elements that defined the theistic schools of the time. It was believed to have been founded some time the 6th century BC, though similar beliefs can be traced back earlier. They believed that truth must be something you can prove via perception or other hard evidence rather than inference. This obviously led them to reject the spiritual beliefs of the time, as that's something you cannot perceive in the traditional sense.
Sadly, much of their texts have been lost over time, so a lot of what we know about them comes from other texts discussing them or refuting their beliefs. Because of this, it can be hard to pin down specifics of their beliefs or lack of beliefs. For example, many texts describe them in a rather hedonistic light however some scholars believe these claims may have been intended as slander.
It's unknown exactly how large their following was, however it had enough of a following among intellectuals to gain a lot of attention and debate surrounding it.
So atheism definitely did have some presence in largely polytheistic societies, however we cannot measure exactly how prevalent it was.