Took me a while to wrap my head around that, but I think you mean the number of humanists is underreported, because they would be more likely to tick “”no religion” than “other” and fill in “humanist”.
My expectation for demonstrating that something has increased is that you compare it to past figures, e.g. the 2011 or 2001 census.
Apparently Shamanism increased from 650 in 2011 to 8.000 in 2021, but the number of Wiccans only increased by 1.000 (to almost 13.000) in the same period
