Currently my parents are in Canada in Visitor Visa and will be completing 6 months here.
After going back to India when is the next time they can come back to Canada, i have heard that they will have to be out of Canada for atleast 6 months to re-enter back...but wanted to confirm.

Thanks in advance.

As far as I know, there is no such rule. But that period is recommended to have less questions for issuance of visa again ( if they do not have visa).

Also if they have over stayed in Canada after expiry of their permit, it is non compliance of Immigration Rules. In such case they are required to stay out side more than 6 months to cleanup the record.