How do I determine solar panel amperage?
You will never get more than 2A at 5V since that is all the controller is rated to give you. 

Whether you get less than that will depend on how the controller converts the solar panel output. 

If the controller is worth it's stuff, and at least 83.3% efficient, you should be getting the 10W (5V * 2A) it suggests when the 12W panel is lit well enough.

But then again.. this is cheap "e-bay" type stuff with no specs..so good luck.

ADDITION

The voltage output from solar panels is very dependent on the load. 6.6V is probably the open circuit voltage. It's the wattage that counts. 

If the controller is up to snuff it will pass up to 10W from it to your output, plus a bit to do it's own thing. 

BTW.. if you need 2A for your load, you will be way underpowered if there is a cloud in the sky with this setup. For decent operation, your panel rating needs to be significantly higher than your average demand.