Well, I think you need the one pair that you're wearing on the day you arrive and two or three pairs for your journey home; so just to be on the safe side and for peace of mind, maybe 4 or 5 pairs.
Just like yourself, I was advised to bring changes of underwear, pyjamas, dressing gowns etc and as I was also advised I'd be in hospital for at least 10 days after J pouch creation, (I'd already had my bowel removed 9 years earlier) I took absolutely everything and I hardly used a thing.
I was actually in hospital for 15 days and pyjamas, underwear even slippers and dressing gown were all provided.
My surgery, was next day after admission; the first night being the only time I wore my own pyjamas and after surgery, I was in a High Dependancy Ward for maybe 3 or 4 days and all I wore was a hospital gown and disposable paper boxer shorts, which could easily be torn off if need be.
I was at St Marks Hospital, London UK, which is a National Health Service Hospital, so I would assume a Hospital that's paid for via a Insurance scheme would be the same, if not better.
Once back on a general ward, I was provided with adult nappies (diapers) to use with my paper boxer shorts and I could use as many as needed; a clean gown was provided daily, so too were pyjamas, in fact, once I was able to wear pyjama, I could just get them whenever I wished from the supply cupboard.
In hospitals and wards that encounter our kind of surgery, they have to have an abundance of clean gowns, pyjams and disposable underwear due to the very high risk of leaks, spillage and accidents and patients can't be expected to take in so many changes of underwear or pyjamas, nor can we be expected to wash items whilst recovering from surgery or be hanging around naked, waiting for visitors to arrive with clean clothing.