You’re allowed 1+1.

If you bring both, one goes up and one goes under.

If you bring one, put it wherever you want (though as a courtesy so someone potentially doesn’t have to gate check a bag, I’ll usually put mine under if it fits).

Unless you’re in a bulkhead row, it’s never 2 up. These are the people that ruin it for everyone. They put their rollaboard, their personal item, and a giant winter jacket up and take an entire bin for themselves.

While many of these things are not written policies, they are considerations for your fellow human beings that you’ll be sharing a tin can with for at least a few hours.