There are ~17M real estate investors in this country who own at least one property outside of their primary residence. So that's the "technical" answer.
More / interesting insights:
These investors are estimated to participate in the purchase ~960,000 transactions this year.
I’m not sure what percentage of these transactions are done by new, first time investors, but I’ll be surprised if it’s less than 30%. So, there's 300,000 new investors per year.
Of the ~28M structures (45M in rental UNITs) in this country that house rental units (most of which are single family rentals, duplexes, triplexes, and quadplexes), 90% are owned by “retail” (less than 10 properties) investors.
60% are owned by investors with just one or two properties.
We do not know how many owner occupied transactions involve “house hacking”, “live in flip”, or how many "second home purchases" are purchased with the intent to invest. But, I bet it's several hundred thousand incremental transactions per year.
On top of that, many people become "accidental" landlords when they inherit property or can't sell a primary for some reason when they need to move.
On top of that, there is a large, unknown, contingent of "investors" who are wholesalers, agents, or otherwise involved in the real estate industry but do not actually own property in a meaningful way.
On top of that, there are millions of households that invest in REITs, syndications, private placements, or invest through other similar partnership structures.
On top of that, up to 30% of the US adult population is "interested in" investing in real estate. That's 50M+.
So, on the low end, you have 300,000 new investors per year. More realistically, we have somewhere between 500,000 and 1M first time investors who are buying through a means other than a traditional investment purchase.
To answer your question in the most unhelpful way possible, I have thus concluded that there are between 300K and 50M people who are serious about real estate investing in this country and would thus classify themselves as investors.
The "right" answer to this question, of course, is 2.8M. That's the number of investors who have created a membership on BiggerPockets, establishing the legitimacy of their intent to invest :).