if(cData >= 20) cData = 0; // keeps the value under 10 values
If the value cData is greater or equal to 20 then make the value of cData equal to zero. As you need two addresses for storing one value by keeping the address value under 20 you will restrict the code to storing only 10 values.
This lets you just keep adding 2 to the value of cData, which is your EEPROM address and it never gets above a value of 20.
As I have told you many times in this thread, but you seem to ignore me, you are not resetting the value of where you write to in EEPROM where as you are resetting the value of where you read from. This is because your reading is in a loop that reads all the values but your writing is not.