Depends on what alternator you currently have, do you have the 90A, 120A or the 150A? It's printed on the back of the alternator and can been seen with the alternator in the car. Most would have the 120A, the 150A is a easy upgrade. You can get one from W210's, W220's, W163's. pretty much anything with a M112/M113 engine. Standard is 120A, 90A in some markets and higher models got a the 150A (inc AMG).
150A is more than enough for most applications with a good battery, anything more than this will be a custom job.