@649spar22, If you take the widest definition of ‘parasite’, which will include viruses as well, then there are more parasitic species than non-parasitic species, so I guess hundreds of millions or something like that. This is because parasitism is a very successful survival strategy.
If you want to know how many of these can infect humans, and this is not counting viruses, I have read that there are about 300 species of worms and 70 protozoa which can infect humans, but I never spent time counting them.
As Franco said, there are pleeeeenty of types of parasites. They might have evolved and become specialists to certain host, or become generalists and able to infect a wide range of hosts. And also many hosts can have more than 1 types of parasites.
To help make sense of this huge variety, people have tried to group them by some criteria. THis can be, ecto- or endo- parasites depending on whether they live inside or outside bodies. Or it can be grouped based on their body shape and anatomy; for examples, parasitics worms can be grouped into round, fluke (they are flat-like-a-leaf type), or tapeworm. Then there are also parasites that are not worms and these are protozoa. They can also be grouped based on their structure as seen under a microscopes.
There are a huge varieties and they used huge range of strategies to survive, and that’s what I like about parasites!