You will need to use some other method of informing your loop when to stop iterating. Hardware designs require fixed sizes for both loops and data structures. Otherwise, how can the size of registers, amount of RAM, etc., be determined?
One possibility is to declare your array a to have some maximum size. Then you can replace your count size 'c' with a data valid strobe coming from your testbench. When the strobe is '1', process the next pair of inputs. When it's low, do nothing.