One big enough to handle the amount of power you are using. That in turn depends on your power line voltage, if using 208v then the 40kw you mentioned = 192.3amps feeding the circuit breaker panel. That gets split up depending on how much power each individual miners uses. For s9 and such (around 1,250w each) you can use a 15A outlet to power 2 miners.

You need to work with a licensed electrician to get the proper power outlets and circuit breakers wired per your area codes. If you do not do this for a living DO NOT just follow internet opinions about wiring. The risk of electrocution or fire is too great.