Can retail investors become millionaires from cryptocurrency?

It is better if u bought and HODL. Understand what is the usage and important of Bitcoin. It is a peer to peer currency. Crypto could become a regular currency, perhaps in the future. So if you have some now, and hang on to them. You could be richer than those now, since u could transact much more easily than others.

Not now or the future. They started investing in Bitcoins shortly after it was launched when it was a couple of dollars per Bitcoin. However, at that point in time, no one would have predicted Bitcoins would hit USD20k per Bitcoin. It has dropped to USD3k since and has climbed to USD8k recently. Whether it will climb back to USD20k is anyone's guess. But at USD8k, you would earn USD12k per Bitcoin, which means you need go buy around 84 Bitcoin to become a millionnaire it if hits USD20k again. The will cost you USD672k. Not really good ROI and high chance it won't hit USD20k per Bitcoin again.