I have found 30 hidden packages in Portland on GTA3 - thats the first island. I don't think there can be more than 30 in Portland as there are only 100 over all, and the second two islands are both bigger.
Oh wait, GTA Unleashed says there are 33. 2 of these however can't be reached until you've opened up the other two islands so thats ok. Not sure where the other 1 is.
Vice City Unleashed is a good resource if you want to find 100 packages in Vice City.