amp/ wire size noob ?

[Avo Mk8]

A 100amp breaker will be installed to feed it, just in case we end up using all of the outlets at the same time.

In that case, at least in the UK, and I imagine true for the NEC, you will need a cable rated at 100A, so if there's a fault, the breaker will protect the cable. So I think that means probably #2 or 3 AWG depending on the cable type.