You can get up to 200 amp altenators,but you'll need to upgrade the wiring to handle that much amperage...
Many stock trucks only had 37 or 42 amp altenators and got by just fine--even on trucks with dual batteries --but when you add high wattage lamps,stereos,winches,etc,the altenator may not be able to keep up with the demand..and a higher output one will help..you can get 90 and 110A ones off other GM vehicles that will bolt up and work with stock wiring..anything larger will need some of the wires upgraded to larger gauge..