Is there a maximum number of times you can stack a PSN membership?

I bought 6 twelve month PSN membership cards.  I applied all cards.  No errors with entering the codes.  I received receipts for them all in email.

After applying all 6 it should have taken me to have an expiray date of Dec 31 2022

However,  the expiry date is only until Dec 31 2020.

Did I just waste two codes?  Can you only do a maximum stacking of 5 years?  Please tell me

thanks