There's all types of geniuses, just as there are all types of intelligences and trades.  A MENSA genius may be a genius at taking a test that tests for geniuses (there's bias in them), but he may not have that intelligence translate into a successful or note-worthy speciality or a job.  In Malcolm Gladwell's Outliers, he cites a study of so-called early childhood geniuses in which the researcher discovered that only about half of them were even moderately successful in the adult world.

There's 9 different intelligences: bodily-kinesthetic, naturalist, musical, existential, mathematical-logical, interpersonal, intrapersonal, linguistic, and spatial.

Einstein, for example, was probably a genius in spatial and mathematical-logical but certainly not in bodily-kinesthetic or interpersonal.

Michael Jordan was a bodily-kinesthetic genius, but not any of the others.  The same can be said for Mozart (musical), Faulkner (linguistic), and Kierkegaard (existential).

Who's a naturalist genius?  Survivorman?

In any question like this, especially if it's an essay title, it helps to define your terms first. So first you need to define your meaning of the word 'genius.' For some academics, this word refers to intellectual proficincy. This is notoriously difficult to measure, but a rough guide cna be represented as an IQ score. Others would argue that there are diffening forms of intelligence and skill. For excample - was Winston Churchill a genius as a wartime strategist and politician? Some would say he was, yet by his own admission he was 'a duffer' at school. How many esteemed and learned men perspire with frustration when attempting to fix something relatively simple like a central heating switch, yet the relatively uneducated Joe Plumber has the hot water and heating working in a jiffy. I suppose you could say genius is 'horses for courses' and there are as many geniuses as there are people who apply the term to another person.

Well, that would be hard to say.  Those with an IQ (Intelligence Quotient) score over 140 make up about .25% of the population, or around one of every 400 people.  That's quite a lot, roughly just under 200 million people worldwide.  But if you mean people frequently referred to as "geniuses" for standing out in their particular fields, there are a great many.  There are many people who are unusually proficient in one field or another who have much lower IQ scores.  In addition, if you restrict your definition of "genius" to mean only the most outstanding few at the top of each field of endeavor there would of course be many fewer.  But specifically going by the accepted definition of the term as used in IQ testing, about one of 400 people worldwide.

Of course, one of the problems with a concept like this is that IQ tests are very slanted in terms of education, age and cultural orientation, so there could be many more (or less) than the statistics of IQ tests in Western countries indicate.