it depends. scaling horizontally is usually more expensive and complicated but can scale more or less infinitely, whereas vertical scaling is cheaper and easier, but has an upper limit.
thus the first time you encounter this question the answer is, unequivocally, scale vertically until you can scale no more. then start adding listeners. also YAGNI as far as scaling horizontally goes. always make one worker do as much work as it can until it drops dead. that’s capitalistic computing (I coined this phrase just now).