How many locks do I need?

I never bothered with any. If the hostel I was at gave me one I'd use it, but if not I had all my valuables with me, so no big loss. I did find most lockers though used a padlock style lock.

The Pacsafe lock is not a really good one.That type is easy to break into.
I would go for a Master Lock, the ones that have a dial. I got one and it's very easy to carry around.
Don' bother with the cable lock for trains. Just keep an eye on your bag and you should be fine.If a thief wants your bag, he/she will just cut it off.
I would never go in a hostel with a lock.
They small locks are very useful.I lock my day packpack all the time when I am out walking around.

Typically on trains locks are not needed. Often you are able to stow your pack at your seat. Europe is much more trusting on the trains. Stolen items normally occur in the station rather than on board.

I've even gotten to the point where I don't bring hostel locks anymore!

Depending on where you are traveling and what time of day or by yourself you might find a train lock a peace of mind, but ordinarily it is okay... think airplane overhead storage. Yeah someone can walk away with your stuff but most of us don't think twice about it.

I can't imagine not bringing a lock for a hostel. Anything that will work for a gym locker will work for a hostel.

While there are a wide range of hostels, petty theft is common. It happens when you are in the shower or out of your room. When you aren't physically with your stuff, keep it locked in your locker.

On the train, it depends a lot on where you are. Since most of us are visiting tourist areas at times when tourists visit I would expect thieves who prey on tourists to also be in the area. Most theives won't focus on a backpacker, there are bigger fish to fry, unless you look like you have expensive stuff. If you're leaving an IPOD touch and a Nikon dSLR sitting on the seat next to you while you nap on the train, you should be worried. Keep your stuff in one bag and keep track of it. If you want to nap, a small cable lock to the rack will ensure that no one can easily grab and run with it.

I don't lock my zippers, so two is plenty for me. When I drive a leased car, I only carry one for hostel lockers.

I'd say good riddance if my stuff gets stolen if left unattended in a hostel! It is a mindset. Why travel with things you can bare to part with? I carry the valuable stuff and everything else can only lighten my load. Sure, I'd be disappointed but worse could happen.

We put a lot of value on our possessions when there is more to appreciate in life than worrying about stuff.