Rather than how many balls, it’s helpful to think in terms of total weight (grams). Balls of wool come in many sizes, from 25g to 200g, so you can’t assume.
You can also use yardage (meters), but that means looking at what other people used rather than working it out for yourself. With vintage patterns, that information is often hard to come by.
The pattern calls for 7 or 8 oz. of wool, depending on size. So you would convert ounces to grams, then order a bit extra. Or you could get one ball to check your gauge and go from there (I realise that may not be economical in terms of repeat postage).
Does the pattern give a tension/gauge? You need to check this as well when choosing yarn, along with converting the needle size.