How many Stringing Christmas Lights can you put up before a fuse blows?

It depends on what type of Christmas lights.

If they are the (now) standard 50 , 100, 150, mini-lights, they use a 3A fuse and you COULD use a MAXIMUM of about 750 lights if they are strung together from one string (end to end plugs) before blowing the fuse built into the plug of the first light set.

If you plug them into separate outlets (or into an outlet-strip), then you could light about 3750 lights on a (otherwidse unused) 15 A household circuit.

With the new LED type Christmas lights, it might be even more.