How many Puranas are there actually and do we have the original ones with us?
Most of them are translated by Western scholars who don't even know Sanskrit.
Of the extant Puranas, eighteen are distinguished as 18 Maha-Puranas, or major Puranas, and an additional eighteen as Upa-Puranas, or supplementary Puranas, though which volumes belong to which group is the subject of some debate. Specifically, the Devi-Bhagavata, one of the principal scriptures of Sakti worshippers, is claimed by its admirers to be a Maha-Purana .....
Windows into the Infinite, A Guide to the Hindu Scriptures by Barbara Powell
It is definite that we do not have the original text for some Puranas.
The Agni Purana speaks of the Garuda Purana consisting of 8000 verses, while according to the Matsya Purana and Narada Purana, it comprised 18000 verses. Prof Wilson believes that the Garuda Purana contains about 7000 verses. It is quite possible to think that the original Garuda Purana, profusely referred to by the other Puranas, is lost to us and in course of time present Purana was compiled.
Preface to the New Edition (of Garuda Purana translated by M. N Dutta) by Professor Pushpendra Kumar