A sofa with no cushions can feel like a waiting room. A sofa with eight can feel like a pile you have to dismantle before sitting down. The sweet spot depends on one thing you can measure with a tape: how wide your seat is. Everything else, including how many cushions, which sizes and which fabrics, follows from that.
Measure the seat from the inside of one arm to the inside of the other. That is the space people actually sit in. A rough guide is about 55 cm of seat per person, so a 180 cm sofa is comfortable for three only if you keep most of it clear. Cushions that sit on the seat steal that width, which is why a big pile looks cosy in a photo and cramped in real life.
The easy rule that follows: let cushions live at the ends, against the arms, and keep at least half the seat open. Try it below. Change the sofa width, pick a cushion size, and tap any cushion to change its fabric.
For most sofas the answer is two to five. A two-seater is happy with two, one at each end. A three-seater usually takes three to five. Beyond that you are layering for the look, and you will probably move them off every time you sit. Odd numbers feel relaxed and casual, pairs feel formal and tidy. Neither is wrong; pick the mood you want.
Layering works because of size steps. A larger cushion at the back, a slightly smaller one in front, and, if you like, a long lumbar cushion (often around 30 × 50 cm) on top. Sizes within about 10 cm of each other look deliberate. Wildly different sizes look accidental.
Texture is what makes a sofa feel soft before anyone touches it. A good starting mix is two or three fabrics, with one that is smooth (cotton, linen or velvet) and one that has visible surface (wool, corduroy or bouclé). Smooth fabrics reflect light and look crisp; chunky ones soak it up and look warm.
Keep colours in one family and let the texture do the work. If you want a pattern, use one, and repeat one of its colours in a plain cushion. Four different patterns on one sofa rarely feels calm.
Cushions are one of the cheapest ways to change a room, and you often do not need new ones. Swapping only the covers on inserts you already own gives a new look for the season. Second-hand covers and inserts are easy to find, and a cover can be sewn from an old throw, a linen shirt or a fabric remnant. If your sofa feels crowded, remove one cushion before you add another.
The planner above is a starting point, not a rule. If you love a sofa full of cushions and you sit in one corner, ignore the half-seat guideline and enjoy it.