Do I still get 20 percent contribution if I earn under the basic rate?

Dazed_and_C0nfused

It has no impact on NI whatsoever.

If they make £12,500 profit they would be liable to pay both Class 2 and Class 4 National Insurance, relief at source pension contributions do not reduce your income for either income tax or National Insurance purposes.

xylophone

Are we saying that a self employed person that earns £12.5k can put £10k in their pension and it be auto boosted to £12.5k, thus getting tax relief when you never paid tax on in the first place? 

See https://www.pruadviser.co.uk/knowledge-literature/knowledge-library/tax-relief-members-contributions/