Yes these are rumours.
1) The codes are normally between 16-18 numbers
If we say the you buy one thats 16 numbers long, you enter it, it gets accepted then that number is basically removed from the operators database. So no possible number can be entered unlimitedly like you have heard.
2) The numbers a generated using a mathematical sequence. oviously this is not going to be a basic 2+2 = 4 sequence.
So if you do want to top up unlimitedly, i recommend taking the Cheif director of you mobile operator hostage and demand he gets u the squence
hope that helps lol
