In answer to Dave Dewey's question, I would say always punch your
own valve pouches.  You will have total control over what you are
using in your instrument.  You can choose exactly the thickness and
quality of the leather, and you will save money.  You can take care
and choose to not use suspect areas of the skin, whereas the ready-made
supplier may not be so careful.  It will doubtless be much cheaper
as you will not be paying for his labour.

I do not know what an Arc punch is but over here we would use a Wad
punch for such a job and these can be purchased for around 25 GBP for
the relevant size.

As for working out how many pouches you can cut from a skin, simply
think of each pouch in terms of a square of side equal to the pouch
diameter and divide the usable area of the skin by the area of the
square.  In real terms you will get more discs if you arrange the
circles 'hexagonally'.

Best regards,
Nicholas Simons
GB