Solar power wattage requirement
Okay, you have 10 chargers that output 5.7V at 0.8A. Ideally we need to know the input specs, as the conversion from 100V-240VAC to the 5.7VDC will not be 100% efficent (usually on the plate there should also be the input ratings also)   

Anyway let's run some rough estimate numbers. Let's assume the chargers are 85% efficient for now.

So the wattage requirement for the 10 chargers input will be:

(1/0.85) * 5.7V * 0.8A * 10 = 53.7W

Now you have to decide how you are supplying the phones charging voltage - whether you bypass the chargers and drop the 12V solar panel down to 5.7V, or use an inverter to create say 120VAC to supply the chargers. The second option will include the inverters inefficiency since you will be going:

12VDC -> 120VAC -> 5.7VDC

If you just step the 12V down, you are going:

12VDC -> 5.7VDC

Inverter Option

There are plenty of cheap 12V inverters available on eBay, so lets take a look at the first option. We already assumed the chargers need 53.7W at their input. Now lets assume the 12VDC->120VAC inverter is 80% efficient, then we need:

(1/0.8) * 5.37W * 10 = 67.2W from the 12V panel.

So current wise this equates to:

67.2W / 12V = 5.6A

Bypass Chargers

For the other option:

12VDC -> 5.7VDC

If we assume the step down converter is 85% efficient, then we get the same result as above with the charger calculation:

(1/0.85) * 5.7V * 0.8A * 10 = 53.7W

And then the panel current would need to be:

53.7W / 12V = 4.48A