Welcome to message forums.
Youll have to bite your tounge at times. LOL.. Or just grow really thick skin.
Heres my view..
15,000 watts is around 1,200-1,300 amps of current draw. In your electrical system you want as much current to come from the alternators as possible.
Your alt setup should be able to supply 100% of the current draw at all times. Even at idle. This is easy when you have 1,500 watts but you have 15,000 so your going to have to rely on batteries to supply the amount of current your alts are not able to.
So in your case you have a suburban... Your lucky in that respect because you can run 4 alternators. If you do an Iraggi Alternator setup like Steve Meade has on his truck you will have around 1400 amps of max current and about 500 amps of idle current. You may want to opt for the lower max higher idle options so you have more idle output. If you do, I would recommend running 4-260's this way you will have 1040 max amps and 720 amps at idle. This would be a little better setup for your amount of amplifier wattage.
With the higher amperage alts and your current draw you will be over by around 800 amps that the batteries will have to supply. This will by only while sitting still. While your driving you will be right at what the Iraggi Alternator setup can do.
So I take the current overage which is 800 amps and divide that by around 150-175 amps per battery. That comes to either 4 or 5 batteries. If you go with the smaller amperage alts like the 260's you could still use the same quantity of batteries even tho the calculations say you can use less..
I also try and use an even amount of batteries in relation to your amplifiers. Makes connections easier and more even distribution to have 3 amps and 3 batteries.
Hope this helps.