So while this answer isn't all encompassing, I think the answer to your question is most likely no.

From what I have come across in general and from various /r/askreddit threads asking Holocaust deniers why they believe that, I would say the answer is no. Usually the arguments are deeply flawed and often cite either revisionist sources or pseudoscientific things (for example, one guy talked a lot about the Leuchter Report).

Admittedly, there are some minor discrepancies that are a result of post-war Allied propaganda. These are often misconstrued by revisionists as evidence that Holocaust a whole is a hoax. However, in reality they are just simply examples of post-war, post-holocaust agendas. Nothing more.

There is nothing of substance to suggest the number of 12 million murdered isn't accurate and there are mass graves in Russia and Poland from the mass shootings done by the Einsatzgruppen (despite Himmler's best efforts).