You can buy your own used Sky+ box(es) cheaply from the likes of ebay, and use them to receive (not record) the free to air channels. You won't be able to buy an additional subscription to enable the recording feature or any subscription channels.
As to the number - the limiting factor is how many spare ports there are for use at the LNB (the bit on the arm of the dish where the cables connect). Each additional box will require a connection to one port. Obviously you'll need to install cabling from the LNB too.
