How many molecules are generally required for cell signallng processes for given cases?
A cell can interact with other cells in zillions of ways. You can send information from one cell to other cells via neurotransmitters, hormones, pheromones, electric signals, magnetic resonance ,leukotrines etc. 

In general a single type of molecule is enough to send such information. Like you require only Acetylcholine(Ach) as neurotransmitter to transmit various nerve impulses. 
But, even for a single type, you require thousands of molecules. Like 1 molecule of Ach can do almost nothing and would immediately be broken by Acetylcholinesterase. You require 1000s of such molecules. 

You can modify the communicating information via different types of transmitters. You can use GABA or glycine to supress any information exchange or use dopamine to enhance it. But again you will need many molecules of GABA or Glycine. 

For visual pathway,  you can use no. of types of transmitters like glutamate, glycine, gaba, dopamine, acetylcholine, substance P etc. 
Neurotransmitters for visual pathway. 

Hormones are transmitters that are required in small quantities. But,  again you require certain concentration. There is normal blood concentration of various hormones like 80 pg/ml for calcitonin. 

Quorum sensing use transmitters like AHLs. Again a certain threshold value is required for them to act. 

Again,  to produce these transmitters you have to go through a rigorous process of transcription, translation and post-translational​ modifications. 
So, for cell to communicate a rigorous process is used.