I offer my view on the basis of running 5 hives, so clearly not experienced in running multiple hives.
BUT - I think I would be asking myself a slightly different question, which would be: "How many hives can I run, based on one day a week, without it becoming a chore?"
I wonder if you might be wise to hold at 50 for another season. Perhaps you could put your additional time this season into increasing productivity per hive, setting up a queen rearing business and/or selling a few nucs? Or trying some pollination contracts? Finding the maximum number of hives for each apiary? If by October you feel you had spare time, then you will know you can increase by a few hives?
This also perhaps spreads out the additional cost of equipment, so you are not left in a pickle if you have a bad year honey wise, or AFB takes out 2 apiaries or whatever.