It is a fire hazard to run more than certain number of bulbs. Sounds like 50íis your light setís limit. Of course, the fire hazard is more likely an issue for indoor applications, but you should never chance it.
This is what is more likely to happen, though, if you run more than the recommended number. I have personal experience here. There is usually a small fuse cast into the plug of the light set. When overloaded, it will blow. I think the fuse is a UL requirement.
Unfortunately, the fuse is usually not replaceable, so once blown, the light set is permanently disabled. You could at that point put on a new plug, but you could really be putting your home at risk.
Someone may make a longer set, I donít know. In my personal situation, I ended up simply running outdoor splitters and extension cords on the hooks that hold the lights onto the house, then plugging the additional lights to it. A little more work, but you will end up with all the lights connected to one source, be it plug or switch, and more importantly, a lot more peace of mind.