I tried to learn to play the violin. 
I have always wanted to try. They are important instruments in many different genres.
How many different stringed instuments are there?
Many! Similar to the violin there is the cello, bass, and viola. The violin is the smallest and highest pitched. 
What would be the easiest to learn, maybe the viola?
The viola is slightly larger than the violin, but I think they would be about the same in terms of ease to learn.
WHich stringed instrument has the fewest strings?  The fewer the easier, maybe.
They all have four strings. I feel like the bass would be easy, only because bass lines in music aren't as active as the other instruments.
Oh, that is a good point. I will look into that.
The violin is a very popular choice. It is played in many non-Western countries like India and Iran.