I use 4 blast furnaces. I just finished building 3 36HP boilers and the turbines and industrial steam engines to go with them. That kept my 4 furnaces running constantly for several hours. Admittedly, I don't foresee a whole lot more need for steel in the future, so now I've just got 4 blast furnaces languishing in my base. However, since massive power production was a perquisite to some of the other builds I have in mind it made sense to go a bit overboard and get the steel asap.