As you have a Canadian website in your user profile, I assume you are asking about the legal requirements in Canada.
Canada's Food and Drug Regulations state the following (emphasis mine):
B.11.133 [S]. Reconstituted (naming the fruit) Juice or (naming the
fruit) Juice from Concentrate
(a) shall be fruit juice that has been prepared by the addition of
water to fruit juice of the same name from which water has been
removed;
(b) may contain juice of the same name, a sweetening ingredient, and
natural pulp, oils and esters of the named fruit;
(c) shall conform to the standards for the named fruit juices as
prescribed in this Division; and
(d) may contain, in the case of reconstituted lemon or lime juice, not
more than 10 parts per million dimethylpolysiloxane.
Thus, it is not explicitly illegal to add more water to the concentrate than you removed from the original juice, as long as the resulting product would be marketable as "juice". The rules for that differ per fruit, but see for example the definition of orange juice. Mostly, constraints are placed on which compounds can be present in the product at which concentrations.
