For a regular three-seater, three to five cushions usually look balanced. Three gives a cleaner, more minimal look, while five feels cosy and layered. More than that can start to look cluttered and actually make it hard to sit comfortably.
If you use three, you can go with two matching cushions on each side and one slightly different one in the middle. With five, you might do a pair on each side and one special, patterned or textured cushion in the center.
Mixing sizes (for example, two larger, two medium, one smaller) adds interest without going crazy. Just remember: the sofa is for sitting, not just for display. If you constantly find yourself moving cushions onto the floor just to sit, you probably have too many.
