I patch until they overlap, or until something happens that I can't patch. The most I remember is 13, and I only gave up on that one because it tore near the valve where I couldn't put a patch.
There's no reason not to. I've certainly put on 100 patches, maybe more, and I've had only two failures. One of those was my fault (some Slime had leaked out the puncture and I didn't get it all cleaned off). I've heard people complain about the weight, or say a patch throws the wheel out of balance, but that's just silly.