How many of the first sets of leaves should die off?
That would depend on the overall size of plant the amount and distance from light in theory only the first leaves will die and usually after they are no longer getting decent light. When is the last time you fed this plant and what kind of soil is it in what lights are you using and the distance? I am going to post a support ticket for you to fill out since I am seeing signs of an issue and the more info you can provide the better and more accurate the advise
ILGM Support Ticket:
What is the strain and type(unknown bag seed, strain name, regular seeds, feminized seeds, auto-flower, etc)?
Indoor or Outdoor? If outdoor, planted in ground or in a container?
Size of space (max height and area, length/width)?
Soil or Hydro? Type of Medium used? System type?
pH? Of the soil or medium (root zone/reservoir/run-off) and of the water and/or nutrient mix that is fed to the plant?
Type and strength of nutrients used? NPK? EC/TDS/PPM levels? Of what you feed it as well as what is in the water by itself and in the soil/medium (run-off), or reservoir.
Temperature? Day vs. night temp or highest and lowest temps? Root zone temps?
Humidity %? Day vs. night
Light system/watts/lumens/FLUX/PAR?
Ventilation system? Size? CFM? CO2? AC, Humidifier, De-humidifier?
Number “weeks/days” from into Season, Vegetative Growth or Bloom/flowering?
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but try to be brief and to the point. Short and to the point questions and/or facts will help us help you 
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