This site has some interesting info. Here's a quote:
"There are 140 different sign languages known around the world, and dozens more to find. Nearly all of them have been developed by communities of deaf people."
http://www.wycliffe.org/training/signlang.htm
I think that Canadian sign language is even different from American sign language (maybe not 100% but different enough that communication is not guaranteed). When I lived in Korea I had a lot of friends who friends who were deaf. Depending on their ages many of them had never been taught Korean and could not read or write Korean, so their sign language was not really even based on their "national language." For those people their signing was different than those who had been educated in Korean, and who could read and write it, and who had been in schools for the deaf where they also received formal sign training. Some of them would sign differently to me than they would to each other. They would sign in what was as close to gramatically correct Korean so I could understand it, but when they signed among each other I was luck to pick up 10% of it.
I was able to use some of my signing when I went back in August, but I've lost most of it, as it has been more than 10 years since I lived there.
Sign languages are actually very rich and expressive. I remember thinking at the time that signing was probably one of the purest forms of expression of thought.