depends on which airline you are flying....
but almost all airlines allow 1 carry on luggage + 1 personal item...
personal item can be small backpack, purse, laptop bag..etc etc
just be reasonable.
I've heard that sometimes if the flight is really full, they WILL enforce your limits and might charge you $$. I read online that JAL did this before and they actually weighed the carry on luggage...coz their maximum weight allowed is 22lbs for the carry on. If you are over, they will charge you money or make you check it in.
when i went back to HK in May, the Cathay ground staff who was walking around the counter said my small luggage is ok as a carry-on (along with a back pack)..but when i was checking-in, the ground staff FORCED me to check-in my luggage because it's a pound over the limit...arggghhhh