Hi Samad
Tough question, many many different opinions.
I single parrot would be much more co-operative, as you would not be "sharing" him
I have only one bird, an African Grey Mishka, now just over two years old.
I have had wonderful results, training her and teaching her to talk.
I know if I had another bird present, the results would be very different.
They require so much time and attention.
Below is a link, which should assist you with your decision.
New Parrot Handbook, The - Google Books
Keep us posted as to which path you choose.
Whatever your decision, we will always be here to offer advise and tips.
Good luck
Take care