Board Thread:Manga/@comment-26495049-20180831081518/@comment-40272219-20190725024842

Here's how it all goes down: There can only be nine titan shifters at one time. A human is either injected with or ingests a titan shifter's spinal fluid to become a Pure titan (aka a mindless human-eating titan). However, a titan can retain their human form and become a shifter if they eat a titan shifter, therefore inheriting the titan belonging to that person. That's where the titans' insatiable hunger comes from. They do not need to eat to survive, their hunger is only constant until they consume a shifter and "complete" the transformation. So no, Zeke can not create more titan shifters because he would have to be eaten for that to happen. He'd only be able to transform one person into a Pure titan and then die in the process of making them the Beast titan.

It is also possible for one person to have multiple titans inside them. Grisha Yeager ate Freida Ress' Founding titan and inherited it, leaving him with both the Attack titan and the Founding titan. Eren later consumed him and gained those titans.