1x power cable required for each MX, 2x power cables required for MX250 and MX450. For US customers, all required power cables will be automatically included. Customers outside the US are required to order power cords separately.
My experience in the US is that a power cord is alway included *unless* you specifically order just one with multiple devices. So if you order 10 MXes and one power cord you well get exactly that. If you just order 10 MXes you will get 10 power cords with them.
Confirmed, in order to reduce waste, in the US only all devices (and/or power supplies if ordered as spares) will ship by default with an MA-PWR-CORD-US, but in all other locations the correct power cord must be specified.