To the best of your ability you should confess how many times you committed each grave sin. The Code of Canon Law is clear on this point:
A member of the Christian faithful is obliged to confess in kind and number all grave sins committed after baptism and not yet remitted directly through the keys of the Church nor acknowledged in individual confession, of which the person has knowledge after diligent examination of conscience. (CIC 988)
If you don’t know an exact number, you can give a best estimate or provide an approximation (e.g., "several times"). If you forget to do this or later remember additional occurrences of grave sins, your sins are still forgiven, but you should remember to acknowledge those sins at your next confession.