How Beast Titan was able to transform multiple people at once?
Spoilers ahead. Be warned.

He most likely used gas which has his spinal fluid.


It was revealed in Chapter 110 how Marleyan soldiers spread gas which contains Zeke's spinal fluid to Ragako village. Zeke called it a Gas Weapon. He explained that any Subjects of Ymir who will inhale the gas won't be able to move and would lose consciousness. It was then shown how all villagers transformed when he screamed. He probably placed people who inhaled this gas surrounding Shiganshina. He then screamed then transformed alongside with them.