They turn up on ebay quite often but you have to keep looking as they go soon!
I think I used 6 packs to make a 4x4panels pen (really you need to have the walls 3 or more panels high) with a shelf of 6 panels and enough left over to block off my fish tank and some wires and under my desk. Be aware packs usually come with a mix of barred panels and mesh panels, so if you want the cage made from all barred ones like I did then you have to buy more packs than you think (the ones I used for bunny proofing were the mesh ones).
I bought 3 packs and then realised my mistake and had to buy more. Its cheaper because of postage to buy them all at once.
If you find the width of the panels you can work out how many you will need, then round up to the next pack so youre sure you have enough. Make sure you take into account ones to support the shelves.