How many training sessions do you think are necessary to improve the trainees communication skills?

it would be different for each individual & depend on their learning capacity & iq . 

communication skills will take sometimes to learned depending on the current ability of the trainee and his learning capacity.

averagly could it be only it take 7 time session .then there would be great impact on the trainee

My personal experience has been that improvement in communication skills would take some time, patience and lots of practice! Hence, the answer to your question would not be an absolute one blanketing all types of audiences. It would depend on the type of learner, current level of communication skills, gap identification and then you may understand what level of training is required. 

I don't think " many " are the right indicator for improving or acquiring 

It's about the content and tools of the training course itself 

Communication is every day activity, you should plan activities or provide opportunities to build their confidence, and at every weekend. tell them about the lacking areas and where one need to be improved i.e. reading, writing, Speaking and listening. 

tell them about movies, discuss with them on it, or let them discuss with themselves, write a short note about movie i.e. how they find it , what is their favorite dialog etc..

Minimum 3 or Maximum 7 session is required to improved the communication skills

Amount training sessions first depends on the beginning level of the student, proper evaluation is therefor a must.

this depend to deep of the course taken and given to candidate we have short course and long course , but i believe is one month is enough and satisfy for company employees