You'll need to work out the area of the surfaces to be painted, length of wall by height (usually around 2.4m floor to ceiling). For the akward bits around stairs, just estimate.
Armed with this info, take a look at the paint you're interested in. All tins have details of coverage on them. Take a look at [broken link removed] for more details.
Leo
Most places will let you return unopened tins of paint, so it's not the end of the world if you overestimate. Just make sure you check before you buy whether they refund unopened returns.