But if you accept their existence you are a polytheist.
Hmmmm... depends on which definition you take.
The Mirriam Webster definition of polytheism is: " belief in or worship of more than one god."
Wikipedia says: "Polytheism is the worship or belief in multiple deities usually assembled into a pantheon of gods and goddesses, along with their own religions and rituals.
Polytheism is a type of theism. Within theism, it contrasts with monotheism, the belief in a singular God. Polytheists do not always worship all the gods equally, but can be henotheists, specializing in the worship of one particular deity. Other polytheists can be kathenotheists, worshiping different deities at different times."
Whereas the encyclopaedia Britannica says: "Polytheism, the belief in many gods."
Now Wikipedia goeson to say that "Polytheism is a type of theism". And defines theism in the following way:
Theism, in the broadest sense, is the belief that at least one deity exists. In a more specific sense, theism is a doctrine concerning the nature of a monotheistic God and God's relationship to the universe. Theism, in this specific sense, conceives of God as personal, present and active in the governance and organization of the world and the universe. As such theism describes the classical conception of God that is found in Christianity, Judaism, Islam and some forms of Hinduism. The use of the word theism to indicate this classical form of monotheism began during the scientific revolution of the seventeenth century in order to distinguish it from the then-emerging deism which contended that God, though transcendent and supreme, did not intervene in the natural world and could be known rationally but not via revelation.
The term theism derives from the Greek theos meaning "god". The term theism was first used by Ralph Cudworth (1617–88). In Cudworth's definition, they are "strictly and properly called Theists, who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things". Atheism is rejection of theism in the broadest sense of theism; i.e. the rejection of belief that there is even one deity. Rejection of the narrower sense of theism can take forms such as deism, pantheism, and polytheism. The claim that the existence of any deity is unknown or unknowable is agnosticism. The positive assertion of knowledge, either of the existence of gods or the absence of gods, can also be attributed to some theists and some atheists. Put simply theism and atheism deal with belief, and agnosticism deals with (absence of) rational claims to asserting knowledge.
So let's take it one step at a time.
Accoring to Cudworths definition of theism, Buddhists are not theists since they reject that all things arise from a single infinite cause. Some may make a distinction and say that those Buddhists that take the Tantra of the All Creating King as scriptural (which all Buddhists do not) are actually theists. But from my brief glance at the text it seems that the All Creating King is merely a personification of ones enlightened state of mind and not a distinct being.
Now if we take theism in its broadest sense we find that Buddhists do fit the category of theists.
Now if we look at the defintions of polytheism again we see that some forms of Buddhism do fit the mould. There are defintiely instances of worship (propitiation) of samsaric deities within Vajrayana Buddhism. Worldly protectors for example. Naga kings. etc... But again, even this does not put the practitioners squarely into the realm of theism because of the lack of the idea of an eternal cause of all existence.
I think we can alll agree that Yidam deity practices definitely do not fall into the category of theism.
So what about atheism? Wikipedia says: "Atheism is, in a broad sense, the rejection of belief in the existence of deities. In a narrower sense, atheism is specifically the position that there are no deities. Most inclusively, atheism is simply the absence of belief that any deities exist. Atheism is contrasted with theism, which in its most general form is the belief that at least one deity exists."
Well, it seems that Buddhists are defintiely not atheists.
So Buddhists are definitely not atheists, they are definitely not monotheists, they are not strictly speaking theists and they are a kind of polytheist.
What about pantheism? Again Wiki: "Pantheism is a word derived from the Greek roots pan (meaning "all") and theos (meaning "God"). It is the belief that everything composes an all-encompassing, immanent God, or that the Universe (or Nature) is identical with divinity. Pantheists thus do not believe in a personal or anthropomorphic god, but differ in exact interpretation of the term."
I would say that there are definitely elements of pantheism in Buddhism, the Kulayarāja Tantra seems to move in this direction though it fails when it gives a human form to this all encompassing divinity.
In closing I would say that Buddhism can be considered atheist when compared to Abrahamic monotheism or to "Hindu" polytheism. Mainly on the basis that the gods in Buddhism are not creators BUT it can be considered a sub-category of polytheism if the creator aspect was dropped. Unfortunately all these definitions tend to revolve around the existence of a creator god(s) - the non existence of a creator god(s) axis and Buddhism does not fit anywhere along this continuim.