Well, the question is asking how many people it will take to give off 100 Watts of power. Since each watt is a joule/second, what you need to do is to find out how many joules of energy each person gives off per second from 14C. In order to figure this out you need a few pieces of information:
1. What energy is released in each 14C decay.
2. How many decays per second occur for each person.
For part 1, the energy released in beta decay is the Q value of the reaction which can be determined from the mass defects: 3.0199 MeV - 2.8634 MeV = 0.1565 MeV per 14C decay. You can then convert that number into Joules.
For the second question, you need to make some assumptions. The ones I made were that each person weighs 75 kg, of which 18% of that weight was due to carbon. I also assumed that the 12C:14C ratio in the human body is the same as it is in the atmosphere or around 1012:1. Based off of that information you can determine the number of 14C atoms in a human body. Once you know how many 14C atoms the body contains, you can determine the activity thanks to the relationship: Activity = Number of 14C atoms * ln(2)/Half-life. You should get something around 2.597E+3 decays per second.
This allow you to determine how much energy a human gives off due to 14C decay each second. Take the 100 Watts and divide it by the Joules/second from a human and you will be able to figure out how many humans you need in order to power a 100 Watt light-bulb.