Take as many dresses or as few dresses as you wish, mindful of the need to store them and (in most colleges) to clear your room during vacations.
There are at least two vintage clothes stores in Oxford where you can obtain second hand finery, helping to save the planet by doing so. One is at Cowley Plain, which is the roundabout on the eastern side of Magdalen Bridge, not far from St Hilda's. Another is on Ship Street, beside Jesus. That one is run by the same charmingly bonkers French lady who was selling second hand clothes to undergraduates forty years ago.
The usual commercial outlets can be found on Cornmarket and in and near the Westgate shopping centre, and there are places in the Covered Market which sell frocks. For swirly boho hippy garments check out Little Clarendon Street, which used to be known as Little Trendy Street (I don't know if it still is).
I doubt that window gazers will be a problem unless, perhaps, you live in a college which attracts Harry Potter tourists, such as Christ Church or New College.
You may recall Cousin Jasper's advice about rooms in the front quad in Brideshead Revisited (advice which Charles Ryder cheerfully ignores):-
Finally, just as he was going, he said, "One last point. Change your rooms." They were large, with deeply recessed windows and painted, eighteenth-century panelling; I was lucky as a freshman to get them. "I've seen many a man ruined through having ground-floor rooms in the front quad," said my cousin with deep gravity. "People start dropping in. They leave their gowns here and come and collect them before hall; you start giving them sherry. Before you know where you are, you've opened a free bar for all the undesirables of the college."
I do not know that I ever, consciously, followed any of this advice. I certainly never changed my rooms; there were gillyflowers growing below the windows which on summer evenings filled them with fragrance.
I wish you a happy and successful time at Oxford.