if it isnt a sealed battery, dont put it in the car, period.
and i always see people running two batteries on there alt and it seems to do fine but they usually have HO alt plans in the future. I have heard of people running 7 batteries on a stock alt, but didnt know how long it lasted either.
personally, i would be confortable with 1 extra battery on a stock alt but if you need more than 1 extra, go HO Alt. but thats just me.