This depends on a lot of different things, and there wont be an exact number I can give you.
Let me start at the beiginning... you will need to get your tank setup and start cycling it (there is a thread/article on here that breaks down the ways to cycle your tank and the process of it if you are not familiar with tank cycling.) I can post a link to it if you cant find it, just let me know.
My advice is to think about what kind of fish you want to keep before you decided on a substrate (as some fish, especially bottom dwellers, can prefer sand over gravel, and this applies if you plan on your tank being planted with live plants or fake.)
The amount of fish you can put in your tank depends not only on your tank size, but your filteration capacity, the types of fish you wish to keep, the fishes' bioloads, and your water parameters. Also, it depends if you want goldfish, or tropical fish. Aggressive or peaceful community fish? Shoaling fish? Lots of small active fish? Or would you prefer a bit larger calmer fish? Are you wanting a tank that is simple to care for, or a tank with fish that may require extra attention? Are you looking for more colorful fish? Or fish with more of a personality (which is up for debate through opinions)?
Once you set your tank up (you can put plants in right away if you want to) and start the cycling process, get a test kit for your water...most popular and accurate is the API Freshwater Master Test Kit (liquid). Test your water. Once you know if your water is hard or soft, what the natural pH is of it...from there we can better help you decide what fish and how many of that fish and what is compatable with those fish.
It all really comes down to what do you like, what will your water parameters be ideal for...and then we can go from there.
I forgot to mention... at the top there is a menu button for "Tropical Fish Profiles" That is a great place to start looking at different kinds of fish and seeing what their needs are from behavior, tank size, food, water parameters, and more.
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