As an example, assume we have the following 3 translation examples.
the dog <-> le chien
the chat <-> le chat
the bus <-> l'autobus
In this case the word the occurs with the 5 French words le, chien, chat, l', and autobus. Thus n(the) is 5. You should set T(f | the) = 1/5 for f = le, chien, chat, l', or autobus.
As another example, the word dog occurs with the two French words le, and chien. Thus n(dog) is 2. You should set T(f | dog) = 1/2 for f = le, or chien, and T(f | dog)=0 for any other French word f.