I can't truly prove or disprove that notion, but I seriously doubt that the number of participants affects your drop rate simply based on the fact that I did numerous FATEs alone and with others involved, and I noticed no particular pattern. If anything, the areas I ended up haunting mostly by myself were some of the most stubborn (conversely, I would occasionally get an atma on the first or second FATE in an area while going solo).

Basically players know very little about how atma works except that the drop rate appears to be miserably low, and any theories about increasing your personal drop rate are just theories at best. Usually people pull these ideas directly out of their asses and post them as if they were factual.