15 years ago,when there was a protest against the bias of the israeli judiciary,there were about half a million.
Given the broad support among Rabbonim for today’s event,and the growth of the frum population,a million is not far-fetched.
The only point that really matters is how many people will be mekadesh shem shomayim,by davvening and saying shema yisroel together-and may we say chacham harozim.A million is immaterial.