> I saw that one wealthy individual had purchased $1 billion of SpaceX at the IPO. Does that count as a retail investor?

There were two individuals who each bought $1B: Ron Baron and Gina Rinehart.

While they are individuals, they executed these billion-dollar investments through their massive corporate entities and investment firms, rather than personal brokerage accounts.

A retain investor is an individual, non-professional investor who buys and sells securities through brokerages using personal funds.