A single bucket with sub-directories is probably the right choice TBH. It's going to make managing things a lot simpler (although I might be wrong, I don't know what your setup looks like :D). I was mostly concerned because I've seen people attempt to allocate a bucket per customer, and then distribute auth tokens for each bucket around.
There are no real hard limits on the number of buckets an account can have, but generally if you find yourself needing more, it's a good idea to talk to someone first!