If you have 40 accounts per person then you are able to dedicate around 4 hours per month, per account. That comes to about 1 hour per week per account. If you ask me that is way too little time to properly manage an account if you are a full-service agency unless you have some very sophisticated tools and ancillary services.
I suppose it is possible to do that if you also have an automated bidding system that is already fine-tuned and tweaked, and you have outsourced ad writing and testing services, and outsourced analytic analysis and reporting and you are simply managing the teams that work on these accounts. However, if you are doing those tasks yourself, you are really doing harm to your clients through systematic neglect.
At my agency we have teams of specialists, one person analyzes the competition and creates a marketing plan and supervises the implementation of the overall marketing strategy. Another person researches keywords, the ad competition, and clients’ website, and then write ads and uploads, ad groups, to the account. Another team will manage the bids using automation while monitoring and tweaking bidding rules. And yet another team will set up tracking, testing and analyzing results, make recommend changes to the strategies, and produce reports for staff and clients, then there is the billing dept that produces invoices and monthly reports for clients, and the creative staff that provides graphics for the image ads.
I think the bottom line is how well does your system produce results for your clients, Are you helping your client dominate their industry? Are you capturing a leading market share for your client? Are you squeezing out the weaker competitors? If you are not doing these things, your competitors’ clients will be squeezing out your clients.