The error is due to this expression which is a non-sense:
this is command substitution where the command is $ARGCOUNT, that is 1. The shell tries to execute the command 1 which obviously is not found. You want indirect variable reference instead, to retrieve the value of $1:
Furthermore a better result can be achieved using the shift
built-in, which shifts the positional parameters by one every time it is executed (the second becomes the first, the third the second and so on...). Useful in a loop where you have to process the arguments one at a time.