Should be able to run *5* rigs if they are only pulling 1050 watts each.
Extension cords to run a miner = VERY BAD idea, even the heavy-duty ones tend to heat up a lot at the connections.
Only one extension cord that'll bring the 30A 240V connection from the dryer socket adapter to the PDU, no other one other than that; and it's a 10/3 gauge wire. I'm keen on safety too, I don't own the property and my insurance covers only my four wall. Thanks for the advice, will monitor the connection once it's set up and gradually put load on it.