How many service techs?

["JW" <golf4nospam@xxxxxxxxxxx>]

I know that service requests are on-demand, but if you were building a
department, how accounts do you think one Service Tech could handle?
Someone told me there was a "rule of thumb" one tech per _____ accounts.
Never heard of a standard like this and wondered if any of you have?  What
do you think is feasible?  The business has been around for decades and
services lots of older equipment.  Thanks for any insights.