How to find out supply of a crypto on exchanges?

If i wanted to find out the amount of crypto left on an exchange, where would i look?

I'm trying to find out the volume of certain cryptos being bought and sold by exchanges to get an idea of the demand and see how limited the supply is. I remember there was a website that had this information (however accurate that is) someone pointed out but can't remember what it was.

I was going to google it for a while but thought I'd ask you guys because maybe you know of more resources and ways of finding out how to get that information.

Also if you could point out that usefulness of using that as a trading method, I'd appreciate it.