How many real estate investors are there in the United States? They
can be *any* type of investor-single family homes, tax liens, raw
land, REIT mutual funds, RE hedge funds, etc.  And the investors
themselves can either be current investors or have been investors in
the past.

Basically I'm looking to get the *largest* possible number that is legitimate.

Also I'll need to know the source(s) from which you glean this knowledge.

Thanks!