You could have made millions of dollars with just $20 if you bought bitcoin 15 years ago and held until now. But that is no longer true, the days of getting rich with small capital with bitcoin are over. Bitcoin's market capitalization has reached over $2 trillion, so expecting it to continue growing at 100% per year is no longer feasible and realistic, let alone making millions of dollars from that small capital.

OP should only invest $20 in bitcoin if he plans to invest more (DCA) and hold for many years, which could help him make a significant return on the invested capital. But if he wanted to get rich with that $20, he should go to the casino instead of investing in bitcoin. Bitcoin won't make him rich even if he holds $20 for the next century, let alone 20 years.