Whats the correct number of fish i can put in a 300g marine tank?

There is no correct number because i don't know what fish you want to keep and i am not listing hundreds of fish you could keep. If you would like to add details ,regarding what fish your interested in i will help you .

There is no "correct" number of fish for a tank. As a matter of a fact, you could have a tank that is completely fishless--nothing but corals and live rock. I, personally, would not suggest this as I think this would be a rather boring tank, but people do it all of the time. With a 300 gallon aquarium, you are at the advantage because you can keep a vast variety of fish and a larger number of them. As a matter of a fact, I would guess that you probably could keep any type of fish that your local fish store can order (Of course, if your LFS is the georgia aquarium, then ignore that). I would like to say that you could even keep a small, benthic shark (If you're an advanced fish keeper), such as an epaulette shark or cat shark, but the dimensions of your tank aren't very good for such a larger-sized animal.

The approximate rule is 1 inch of fish per gallon. So technically, you could have 300 guppies each one inch long, 150 fish that are 2 inches long each, or mix and match the numbers. If you want plants or obstacles such as rocks and castles for your fish, I'd recommend less (if it will take up a lot of space) because they need some room to swim!