You can get away with just one light on the background for a head shot, but anything larger and you need two in order to get the background evenly lit all over.
Lots of threads here on shooting white backgrounds. Basically, treat the sitter and the background as two separte subjects, and aim to get the white just brighter than the main subject light. Aim for half to one stop over exposed, no more. Blown to white is pure white, and any more just bleaches the subject outline and creates flare.
If you are shooting a wide area, maybe a full length shot or couple, getting the background right is very difficult. It is better to mop up any grey bits in post processing rather than blitz it with light.
Get some practise. If it's anything larger than a basic head shot, you probably won't get it right first time and will need to experiment to get the optimum positioning, angles and levels - it's always a bit of a compromise.
