If a creature with a +1/+1 counter on it dies after receiving too many -1/-1 counters, it dies at the same time that the counters are removed, and Grakmaw's second ability triggers. Similarly, if Grakmaw dies after receiving too many -1/-1 counters, use the number of +1/+1 counters it had prior to receiving the -1/-1 counters to determine the value of X.

Grakmaw's second ability puts just one +1/+1 counter on it, no matter how many counters the dying creature had.