At work I am constantly helping people pick out new sofa pillows and the question I always get is "how many do I really need?". Honestly there is no simple answer to this question as there are no rules in design. Most people do not want too many pillows to overcrowd the sofa and then not have enough space to sit down, this is totally practical and I feel the same way. But some people like more than enough pillows for a full and layered look (and this is how we set most of the sofas in the store). So it really depends on your personal preference, sofa size and style of your space. But I do have many suggestions for what I usually find works best!
Regular sized sofa with two pillows (a 22" and a 20") on each end and then a throw blanket on the chaise looks great!
An oversized sofa can handle up to three pillows on each end! The sizes are the same on each end (a 24", a 20" and a 16"x24" lumbar) but the patterns are different to add more interest!
With two large sofas, choose the same arrangment on each sofa but have them mirror each other! With three pillows on one end and two pillows and a throw on the other end these sofas feel perfectly balanced!
For a simple combination on a small sofa, two 20" pillows one 18" pillow and then one throw are the perfect amount for comfort and practicality!
Any time you do a grouping of three or more pillows on one end try to mix it up with different sized and shaped pillows, including at least one smaller pillow.
When styling a small sofa or loveseat start simple with one pillow on each end, maybe add in another small pillow to one end or a throw blanket depending on the size.
If you have a large sectional you should put three (maybe four) pillows in the center corner and one or two on each end. If you like lumbar pillows, use them on a sectional they look great right in the middle or in the corner grouping!
In a room with two different sized sofas (large sofa and loveseat, sectional and sofa or even a sofa and chair) you should have noticeably more pillows on the larger one or at least larger pillows.
On a chair be careful not to get a pillow too large, a small chair can easily look smaller with a pillow that is too big. Only on an oversized chair-and-a-half would you consider more than one pillow. If you have two of the same exact chairs in a room, choose the same pillow for each.