In a nutshell, I can run standard 2/10/50 car battery chargers from my generator.  How many batteries can I charge simultaneously with one charger?  I'm GUESSING --because I don't know-- that the best answer is ONE.  I'm also making a semi-educated prediction that other factors would come into play in multiple charging scenarios, like matching battery capacities, chemistry, etc.  

BUT THEN.....

I look at my 48v golf cart that is charged by a single 120v charger for six, 8v batteries...so there must be a way....

My reasoning behind this is... in a grid down / SHTF situation, fuel to run the generators would be limited, and you'd want to get the best results for the fuel expended, so tell me how to do this.....

NOTE... Not looking for "solar" answers.....

I have several 30-100 amp  chargers that can charge 3 batteries at once. Check out Donrowe.com.

I charge all four of my batteries at once. I try to keep the charge rate at 8-10 amps per battery if I have a way to select.
I leave all the batteries connected (wired at 12 volts)
Positive to positive terminals and negative to negative terminals on the batteries.
From the charger I hook the positive jumper on one battery and the negative jumper to the fartherest battery from the positive.
If I have a dire need for faster power I will turn up the amps to charge at the expense of battery life.
That's how I do it. Others have different ways I'm sure that may be better.

This is my SHTF charger that can run on gasoline, propane or natural gas. Its not for everyday use when I need power back in my batteries. It's for emergencies where I need power in the batteries quick. Any other time the batteries stay on a trickle charger.

Neat setup!  

Don't try to charge in series as may be implied by "golf cart" unless the batteries are actually used in series.  Yes, I can see the photos are parallel charging.  

The impedance of batteries is altered in use an if not discharged evenly, they will not charge properly.  Another thing to consider in series charging is that it only takes one bad cell to make it impossible to properly charge the stack. 

You can wire multiple batteries in parallel, positive posts all connected and negative posts all connected, resulting voltage is still 12VDC.

But that won't save fuel because you are now charging a battery many times larger.  So you are pumping more total energy into the bank.
If it takes 5 hours to charge one battery, it would take 20 hours to charge 4 in parallel at the same charge rate (amperage).
Plus additional losses.

The best way to maximize fuel and charging is to run the generator at roughly 50% load which is usually the fuel max efficiency point.
Figure out what that is.  For a 1600W genny (EU2000) it's ~800W.  At 12VDC 800W is 66.6A.  (6.6A at 120VAC).
Pumping 60A into a battery requires a large capacity battery.  A minimum of 120 Ah (AGM).  A lead battery needs to be charged at about 0.1C (which would be 600Ah at 60A).
They're also expensive ~$500.

A 2/10/50A charger is really only capable of putting out a sustained 10A.  
So if you make a large battery bank by connecting multiple batteries together, you're just going to have to run the generator longer.
A partially charged battery is not a good thing.  Better to fully charge a smaller number of batteries than to partially charge several.

A 10A charger is pulling about 1A (a little more) on the AC circuit.

It would be best to run a separate charger for each battery if you are tied to that style of charger.

A dedicated 12VDC charger, like that posted above, is the best fit.

I habe these Samlex I use for 12 and 24V banks. I don't have a 48V charger/inverter yet.

Nice unit.

I have a similar Cotek 24V 25A charger.

you should charge batteries at 10 percent of their rated capacity, a 100 amp hour battery should be charged at 10 amps, so depending on how many batteries you have you can adjust your charging rate, slower is better,  i use a fully automatic 30 amp charger to keep my 2 100 amp deep cycle batteries charged.  i keep them on the charger all the time  they tend to last about 5 years . i used them to run a 1/2 hp. sump pump when the power is out.  last time they ran the pump for 3 hours.  the power came back on and the charger automatically started recharging them.  they were up to full charge on about 4 hours, using a fully auto charger is a nice way to keep your batteries topped up i check them weekly but have never had to do too much to them.  the battery charger automatically sees that they are fully charged.  also the last 10 percent fo the battery charge takes longer than the 90 percent charge.  slower is better to saturate the electrodes with charge.

this site will give you  a lot of info about batteries and charging:

battery university

battery university is a great site.

small generators and iron battery chargers are very inefficient as the peek voltage is low and impedance is high, you must use an electronic charger or a generator over 4kw.
you should only charge similar chemistry and age batteries in parallel.

Those 8V batteries are all connected in series - Which means that they all share the charging current equally - Which means that they all get recharged at the same rate.

Effectively, they are one 48 volt battery.