Without an electricity rate, it's impossible to tell you if you have a chance of making some money...
Personally, i'd suggest you to find your electricity rate, make sure the cabling to the garage can handle the power draw (aswell as the power breakers).
20 S7's will consume 20x1210Watt + a switch + lighting + some fans... I guess around 25 KiloWatt power draw would be a reasonable assumption. This means that a normal household circuit might not suffice. Also, you might no longer be able to get the residential rate, since 25 Kw * 24 hours * 365 days = 219.000 Kwu per year... In comparison, i use about 6.000 Kwu/year for the complete electricity consumtion of my house (me, my wife, my kid, electrical water heater, additional electrical heating of some rooms, wife at home 5 days a week).
For example, a 20A breaker on a normal household circuit seems to allow about 4,6 Kw draw (i'm not an electrician, i found this number online).
Also, this means that the heat produced in the shed will be about the same as 12-13 2Kw electrical heaters... Natural cooling might not suffice.
Before you attempt to do anything, i would also suggest to buy a couple old S1-S3-S5's and see if you can get everything setup and keep everything running for a couple of days.