There are a total of 100 hidden packages in the game, 40 in Portland, 30 in Staunton Island, and 30 in Shoreside Vale. Besides getting $250 when each package is collected, the following reward will appear in your safe houses when you collect the required number of packages:

Handgun: 10 hidden packages
Shotgun: 20 hidden packages
Body armor: 30 hidden packages
MP5 gun: 40 hidden packages
Python gun: 50 hidden packages
M4 gun: 60 hidden packages
Laser aimed sniper rifle: 70 hidden packages
Flamethrower: 80 hidden packages
Rocket launcher: 90 hidden packages
$50,000: 100 hidden packages