Christmas lights - how many strands can I connect?
If the strands are connected "in series" - strand A plugs into an outlet, strand B plugs into the end of strand A, strand C plugs into the end of strand B, etc. then all current for all of the strands will have to flow through the wires of strand A, which are probably too small to carry the current of more than two or three strands.  If you have eight strands connected "in series", the wires in the first couple of strands will likely overheat and melt the insulation.