I contributed 20k in 2018 RRSP, however I would like to carry forward 5k into 2019. I want to buy my first home, can I use the 20k or only the 15k reported in 2018.

As you actually "contributed" the entire 20k to your RRSP, you should be entitled to use that entire amount for your Home Buyer's Plan withdrawal. CRA will have record of you contributing the full 20k, but carry forward 5k of "unused contributions".