to know for sure and get the correct number you should check to see whats the amp rating of your alternator then use a amp meter to measure your car at full power load (eg. turn the AC on, all the lights, the high beam, wipers, music, hold the break down, just turn everything in ur car that uses power on, on full blast) measure the amp draw then minus that from your alternator's rating then you will know for sure how much you have to play with, keeping in mind that you will need to leave 10-15amps to keep the battery alive.
for me, my car came with a 105amp alternator from the factory but i upgrade it to a 200amp PMA and the car can only draw a maximum of 67.8AMPS so i have alot to play with.
if you have anymore questions pm me because i don't look at all the threads everyday.