Hi! You might find some past answers from /u/lcnielsen of interest in the Zoroastrianism section of the FAQ.

This is actually an extremely complicated question because depending on what Zoroastrians you ask, some will tell you that it is not, nor has it ever been. u/asdjk482 offers a good explanation of the very basics of the issue, though I do have some corrections/addendums to add to that answer. I'm going to let this answer run long too because I think the context of what the polytheistic image of Zoroastrian looks like, and where the monotheistic claim comes from are both important here:

Polytheism to c.1700

The most obvious missing detail is that any interpretation of Zoroastrianism is at least slightly "dualist," with all good things stemming from Ahura Mazda's creation of a perfect universe and all evil stemming from Angra Mainyu, the Spirit of Destruction, who seeks to corrupt Creation. Whether Angra Mainyu is equally primordial to Ahura Mazda or one of his creations gone astray differs from text to text and theologian to theologian.

Strictly speaking, it would be more accurate to say that Zoroastrianism developed out of Iranian polytheism, as conventional historical linguistic analysis of the Gathas, a section of the Zoroastrian Avesta scripture written by the prophet Zoroaster or his immediate following, places the origin of the faith c.1200-1000 BC, after the Indo-Aryan and Iranian linguistic groups split from one another.

The Gathas and just one other hymn were composed in the Old Avestan stage of the Avestan language, while the rest of the Avesta was composed in the Younger Avestan dialect from c.1000-400 BCE. The two dialects and the hymns/prayers composed in each present an interesting theological quandary. The Old section is small and narrowly focused on Zoroaster's preaching, so may not represent a very thorough picture of the whole movements' beliefs or even all of the prophet's teachings. However, they only emphasize a few divine beings by name, primarily Ahura Mazda and the Amesha Spentas: a group of six spirits/divinities/angels/gods (more on that below) who represent a series of abstract concepts seen as fundamental to Ahura Mazda's created universe and Zoroastrian belief.

However, the existence of other divine beings is still referenced. The Gathas refer to Ahuras and Daiva. Ahuras are presented as good beings worthy of worship, while the daiva are false gods worshipped by the surrounding peoples who refuted Zoroaster's preaching and attacked his followers. Based on the Gathas emphasis on the evils of hostile warfare and cattle raiding, some scholars have suggested that the Daiva were prominent war gods, but since only two are named, and neither is known outside of the Zoroastrian tradition, it's impossible to confirm that theory. Angra Mainyu, the chief opposite of Ahura Mazda in later texts is notably absent in the Gathas.

As completely abstract concepts, its hard to interpret how much the Amesha Spentas and Daiva were intended to be understood as literal beings or in a more metaphorical sense. The Younger Avestan hymn to Ahura Mazda, the Ormazd Yasht, identifies several Amesha Spentas as part of Ahura Mazda himself, while other hymns from the same time period present them independently. In lieu of any actual names, its hard to say what exactly the plural "Ahuras" were supposed to entail either.

The much larger corpus of Younger Avestan introduces the concept of Yazatas, literally "those worthy of worship." Depending on what names you count, extant Zoroastrian literature names about 53 of these beings. Of them, only Mithra and Apam Napat are given the title "Ahura" (literally "lord"). Both were also very important parts of pre-Zoroastrian traditions. Unlike u/asdjk482's implication, they are never presented as an explicitly triad in the Avesta, they are just three important divinities with the Ahura title that map closely to the later Persian triad of Ahura Mazda, Mithra, and Anahita. Anahita may actually have grown out of an epithet or companion divinity for Apam Napat, "Aredvi Sura Anahita," meaning "Wet, Powerful, Pure." I recently wrote a very long answer about how exactly that Persian triad formed.

Zoroastrian literature, especially the medieval Bundahishn, also provide more Daiva's names, with a total of 43. There are potentially more names if you include non-Zoroastrian literature with Zoroastrian legends like the Shahnameh. These medieval texts actually emphasize the importance and independence of the Yazatas and other good divinities even more than the actual Avesta. Likewise, ancient sources from pre-Islamic Iran contain references to even more gods not named in the Avesta. Not just Darius the Great's "other gods who are" and "gods of the royal house" (see Inscription DPd), but explicit names of both Iranian and non-Iranian gods. See WFM Henkelman's The Heartland Pantheon. Neither Zoroastrian nor outsiders accounts suggest that the Amesha Spentas and Yazatas were less venerated or that the Daiva were less hated as late as the 17th Century, for example the The Dabestan-e Madaheb and the Farsi Rivayats.

Zoroastrianism's status as a monotheistic religion seems to be complicated, and depends partially on the historical era in question.

The religion developed out of an older Indo-Iranian substrate which was certainly polytheistic, and Zoroastrianism (especially in its earlier periods) recognized numerous divine beings - the claim to monotheism is due to Ahura Mazda's supremacy among gods, rather than him being the only god. I suppose it's actually more like henotheism than monotheism, although still not quite because the Ahuras and Yazatas are all venerated.

The Younger Avesta presents an explicit pantheon in the form of the "Ahuric triad" - Ahura Mazda, Mithra, and Apam Napat.

To give an example of another popularly worshiped deity, there was a water goddess by the name of Anahita whose cult seems to have had connections to the ancient Mesopotamian Ishtar/Inanna syncretization complex, according to William W. Malandra in section 9 of "An Introduction to Ancient Iranian Religion".

Artaxerxes II referred to her in an inscription at Susa: "By the will of Ahura Mazda, Anahita, and Mithra I built this palace. May Ahura Mazda, Anahita, and Mithra protect me from all evil." (note that in this triad, Anahita replaces Apam Napat - both are water deities)

Similarly, Darius' Behistun inscription calls upon Ahura Mazda and "the other gods who are". For those inscriptions, see: http://www.avesta.org/op/op.htm

She's also attested as being highly regarded in the Hellenistic period; see Mary Boyce's "A History of Zoroastrianism vol II," around page 211, being conflated with Artemis under interpretatio graeca