A single player must have cast two or more spells during the previous turn for Call of the Full Moon's last ability to trigger. If multiple players each cast just one spell during that turn, the ability won't trigger.

Call of the Full Moon's last ability will look at the entire previous turn, even if Call of the Moon wasn't on the battlefield for some or all of that turn. For example, if you cast Call of the Full Moon and another spell on the same turn, you'll have to sacrifice Call of the Full Moon at the beginning of the upkeep of the following turn.