Short answer: thousands.
For that, we must define "celtic". All modern folklore aside, fom a mere archeological point of view "celts" were the oppida building people, referred to as "Keltoi" by the Greek ~500BC. Archeologists usually mean the late pre-Roman iron age people when they speak of "Celts" in general, with the eponymous find site being La Tène in Lake Neuchâtel. Though we don't say "culture", rather "time" or "period" because culture has a different meaning for different people. La-Tène time spans ~400 years (~450BC until Romans).
But that definition is not allways followed strictly. Often times, Hallstatt (early Iron Age in Europe) is included in "celtic" times.
Some of the best researched oppida are Bibracte (up to 30,000 inhabitants when the Romans made their debut).
https://en.wikipedia.org/wiki/Oppidum_of_Manching (maybe 10,000 inhabitants)
and a list of many more can easily be found when searching "Oppidum". Some of them were surrounded by really huge earth works or situated on spurs in the landscape, fortified by "Murus Gallicus", a terminus technicus coined by J. C.
Sorry for all the Wikipedia, hope the contents aren't that far from archaeological evidence. Can provide more info when necessary.
A note on sources:
It is not trivial to find something in English and iron age is not my specialty, but I found a recent paper on other types of settlement (not enclosed as the oppida) around late La Tène and in the orbit of Bibracte, for the archeologically interested. (Sure, an iron age guru could easily link to more information on the matter). It is better than the Wikipedia stuff, which serves as a rough outline at best.