Phoenix Whipsnade have a good sized herd in a smaller open range zoo than we have so I would be surprised if size was an issue?
Whipsnade's are not really a 'herd'. There is a bull (not the father of the two calves) two cows and two calves. Although the two females grew up together, since they bred the two cows and calves now seem to be always kept apart.
The 'Rhinos of Nepal' exhibit comprises the new indoor house, 3 large stalls with heated bathing moats, leading to the two large outdoor paddocks, each used by one female and her calf. The smaller of these two paddocks also connects to the primitive old building and yards which even now, still seem used most of the time. The bull 'Hugo' (not of Nepalese origin) currently appears to be kept in the old original paddock and part of this house. My guess is this is because while there is a third stall available in the new house, while they are split like this there is currently no paddock for him- unless they divided the bigger one in two. Its not a very satisfactory arrangement really.
So you can see even with just 1.2 (+2 calves) animals, there is a lot of seperate accomodation needed, at least in this case. Most other city zoos in Europe which keep Indian rhino also normally work on the 1.1 or 1.2 basis too. But as we've discussed before, San Diego WAP really do have a big 'herd' though I am not sure if their husbandry differs a lot from Whipsnade.
But it seems to me the open air Western Plains Zoo would be well set up to accomodate several Indian Rhinos in the longterm future e.g. if/when they start breeding. Indian rhinos are not really difficult to breed. What you need are compatable mature animals, and most important, masses of space for introductions and courtship behaviour to take place smoothly. If they can breed successfully in cold European zoos, I'm sure they will in Australia's climate...