How much am I charged for one conference call?

You will be charged on a per person per minute basis. So it depends on how many people you add and how long you talk to them. 

For example, if you call 5 people and talk to them for 5 minutes, and the call rate is 19p/min for 1 person, you will be charged 19p x 5 x 5 = Rs 4.75