I'm playing as Ottomans. I'm not in any coalition or alliances, but I have vassals. I'm the target of a coalition of Venice, Poland, Bosnia, Persia, Ethiopia, Oman. I have fabricated at least 1 claim per each of those counties. I declare war with conquest war goal. Even if I occupy every enemy province I have claims on, in the peace treaty I can only demand provinces from the one country I declared war on. They entered the coalition at different times so the truce prevents them from fighting me all at once. If I declared war on Oman, then Persia and Venice would come to Oman's aid, but Poland, Bosnia, and Ethiopia cannot help Oman because of their truce with me. I thought I would demand few provinces from Oman, few from Persia, one from Venice, all occupied by me, but in the treaty screen I can only demand from Oman.
and this is odd, because few decades ago, when the coalition members were Mamelukes, Oman, Qara, Georgia, and I had no vassals but I had allies in Tripoli, I could demand provinces from numerous enemies at once. Didn't matter who I declared war on.