The speculated number depends on your definition of "domain investor." Boiled down to its rawest form, a domain investor is an entity who purchases domains and resells them for profit. With that in mind, realistically, how many are there? Probably over 100,000 I'm guessing. Can I justify my guess? Sure:
When I started out, 5, 6 or so year ago, I was a teenager who heard about domains being a "good source of income." It didn't take me long before I met many other teens and kids (some as young as, maybe, 12-14) who were in this to make a buck. Even these days, I find kids on NP and elsewhere trying to sell a random 4-letter .com for $200 or so.
A guy I know, aged 19 (I think), asked me "what do you do for a living?" I told him I'm a writer and I occasionally dabble in domains. I told him I don't get rich with domains but I make a bit every now and then. He then became interested in domains, seeing it as a way to help support a lifetime that he and his g/f hope to live together.
It all just shows me that people of any age can get drawn into investment -- with dangerously high hopes. Kids get into this wanting to make a quick buck and early adults, thinking this could replace hard labor, jump in the same. I think there must be at least 100,000 domain-ers out there, although less than 5% have ever made more than $100 from it. And from my guesses, I'd say 60-80% of domain-ers are under the age of 25 -- these make up most of those domain-ers who have yet to get a $100 paycheck.