How many spacers?

Any idea how many cassette spacers I would need to convert my 7spd geared bike to single speed? If that makes sense. I presume I will need to strip 2 cassettes. This seems such an obvious question but just in case I'm missing a trick.

Also is it ok to use one of the old cassette sprockets for the conversion or should I buy a dedicated ss sprocket?

How many? Depends on the chainline of the chainring...SS conversion kits come with lots of different sized spacers, enabling you to set it up correctly.

Cassette sprockets are designed to easily shift the chain from it onto a higher/lower sprocket - I would rather use a non-ramped sprocket.

Fairly sure you'll find that the sprockets and spacers of your cassette are different widths, so you can't just replace a sprocket with a spacer. You can get single speed spacer kits (that include a sprocket) to fit 8 and 9 speed hubs, think I've got one knocking around somewhere, this may well be a better solution that cannabilising a pair of old cassettes. Might be a problem with the width of a 7 speed cassette free body being different to 8 and 9 speed cassette free bodies, but according to Sheldon Brown,

"Any 7-speed Shimano Hyperglide Freehub will actually work with 8 sprockets, without any modification! What you need to do is to use 8 of the sprockets from a 9-speed cassette, with the 9-speed spacers.

To make this work, you'll also need to use a 9-speed chain and shifters. Your old 7-speed derailer should work OK if it isn't too badly worn. The limit stops on the derailer will cause the useless 9th position on the shifters to be locked out, so this will work as a perfectly normal 8-speed rig."

So I'd guess you could get a 8/9 speed spacer kit to work with a 7 speed hub. Hope that helps :)

It's hit and miss depending on your chainline required and the type of spacers you're using.
ie. an on-one spacer kit with two solid tubes will require.. two solid tubes! :)
Whereas stripping an old cassette down is a case of throwing them in until your chain is straight.