I served for many years as a clerk under several bishops (some of that time was before LCR: MLS over dial-up in those days). Even before online donations, I never felt the need for more than 2 clerks in our average sized ward. I always recommended to the bishop that he call one clerk and one assistant clerk and we could then back each other up for illnesses, vacations, etc. I can imagine scenarios where more than two would be useful (wards with lots of transition, wards without access to online donations or many members who don't use online donations, health issues with a serving clerk or assistant clerk, etc).
However I think the worst thing a bishop can do in this regard is call an assistant clerk and not give him anything to do (like another ward did with my son-in-law). Every ward is different and the number of assistant clerks should be considered under inspiration and in full awareness of the actual workload so they feel useful.
But, this is all my personal opinion based on my experience and personality (one of my bishops used to call me "Radar" because I always tried to anticipate what reports he would want and what questions he might ask in meetings so I was prepared to help in the discussions). I provide it just as another point to ponder.