Here's the honest answer: 3 to 4 colours, one pot each. That's the sweet spot for nearly every room.
The mistake people make is either buying one pot of one colour (and then panicking when it looks wrong on the wall) or buying twelve and turning the hallway into a patchwork quilt. Neither helps. Three or four well-chosen contenders give you something to compare against — colour only makes sense in relation to its neighbours.
One pot per colour is plenty. A standard F&B or Little Greene sample pot is around 100ml and covers roughly 1m², which is enough for two coats on two A3-sized patches. And two patches matters: paint one near the window and one on the darkest wall, because a north-facing room will drag the warmth out of anything. Setting Plaster looks gorgeous in bright light and can sulk in a gloomy corner — you need to see both.
If you're torn between very close shades — say Cornforth White against Skimming Stone, or Pointing against Wevet — get both and patch them side by side. That £6 comparison saves you a £200 mistake.
For larger projects (whole-house schemes, open-plan spaces), bump it up. You'll want pots for each room's main contender plus the trim and ceiling shades so you can see them together. But you still don't need backups of the same colour.
Two practical tips. Don't paint straight onto the wall if you can avoid it — use a sheet of white A2 card or lining paper, paint two coats, let it dry fully, then blu-tack it up and move it around the room over a couple of days. And buy in the same finish as your final paint where you can; estate emulsion and modern emulsion reflect light differently, so a matt sample won't tell you how an eggshell will sit.
Shortlist hard before you buy. Four pots, properly tested, beats ten bought on a whim every time.