Ed, Edd n Eddy's Big Picture Show (2009)

Answer: First, you'd have to figure out how much light production you would qualify as being sufficient to consider the fireflies in a jar a "lamp." Lights are often rated in footcandle, lumens, or lux. Without getting into what all that means, you could probably hike a path in the dark with just 25 lumens, which is a little less than 2.5 footcandles. (By comparison, a bedroom may have 20-50 footcandles but an outdoor parking area may only have 1-5 footcandles and street lighting may be 1-3 footcandles). With that in mind, it also depends on the species. The most common firefly in the US produces 1/40 a candle. So 40 fireflies for 1 footcandle and 100 to have a "lamp" to hike with. It should also be noted males light every 5 seconds and females light ever 2 seconds, so you would need more if they're not all lighting up at once.