If your dominant pattern is organic, like a floral, it is usually good to contrast it with a geometric like a stripe or small print. If it’s a stripe, the reverse is true. Just be sure that the scale of the two patterns is different enough that they do not compete. Your eye should have no difficulty distinguishing between them.
This can also apply to rugs. Patterned rugs can greatly enhance patterned upholstery, as long as the two contrast each other in color and design type (i.e., a jaquard floral sofa on a rug with a center medallion.) Additional patterns (yes, keep adding) will follow the same guidelines, pulling out contrasting colors from the dominant pattern and adding layers of interest with complimentary checks, toiles, animal prints or whatever. Your eye will tell you the amount that is right for your personal style.
