Short Answer: Yes! The number of light strings that can be run in series is limited by the types of bulbs on the string, how much power they use, the length of the string, and the type/gauge of the wire used in the manufacture of the lights.
Longer Answer: For pre-wired Christmas tree lights and light strings sold with an Underwriters Laboratory (UL) the maximum power added together of a series of lights plugged end to end is 210 watts.
For example, if a string of lights is rated at 50 watts then only 4 sets of lights can be run in series.
So, an LED light string with a rating of 2.4 watts can be run over 80 sets in series.
Exception to this Rule: This rule does not apply to traditional C7/C9 bulbs and cords (the style you might remember from your childhood). These cool retro style Christmas lights can only be run a maximum distance of 250 feet if they are constructed with 18 AWG/10 Amp wire.
Shorter 25-foot 20 AWG cords are generally spec'd to be run only two cords in series.
Conclusion: Yes, you can only run so many light strings in series. Be sure to check the manufacturer's specifications of any lights you are shopping for and make a note of that number on your Christmas lights inventory spreadsheet after the purchase.
Have other technical questions? Post them in the comments below!