Wow. There will be many. So many.

I have no idea how many though.

Now the average mass of all of them is 17.7 g/mol

So 500 g/mol is a whole lot. About 28 Atoms if you just assume that all atoms of the molecules have the average mass in every possible combination.

Which is a totally unrealistic!

Since there are 6 different possibilities and you assume that it is possible to put 27 atoms (since one has to be C) or less of the same kind into one molecule

which is totally unrealistic,

you get 6^27= 1 023 490 369 077 470 000 000 possibilities. Note, that all molecules with less then 500 g/mol are not even included!

Of course, I am not saying that the number I just wrote down is in any way even a good guess for the range.

But imagine all the carbo chains with subsitutes and all the different possibilities and all the benzene rings and all those amino chains... Just look outside as you think about this and spot a tree. A plant. A bug. Your skin. The earth. The different colors of flours, food and particles. Man I guess the number might just be higher than the permutations of a Rubik's cube.

Did I say Rubik's cube? Oh were are in the Off Topic section, sorry.

EDIT: I thought again, and I come to a more specific conclusion: About 4. +/- 1. Yes. Pretty sure that's it.