This is a question that I have heard from new hobbyists a lot throughout my career in fishkeeping and it is a fair question! I can remember feeling confused myself when I started out in this hobby! The truth is after a few weeks or months you will get a feel for how much you should be feeding. People usually say ‘just a pinch’ or ‘as much as they will eat in a few minutes’. I would recommend thoroughly researching the fish species care requirements before you purchase your new fish. This will let you know what they should be eating, for example a pygmy puffer fish will not accept flake food and will need to eat frozen food or small snails.
If we are talking tropical flake food and you have a ‘normal’ Tropical tank with around 10 or more fish, then yes, a small pinch once a day should do the trick. You want to make sure all the fish get something to eat. Check after a few minutes to ensure no food is left uneaten in the tank, it is wise to remove this if so, as uneaten food can pollute the water. Pro tip: If you place your pinch of flake food underwater and let it go it will move around the tank and make it easier for all fish to get something, this is a much more natural way for most fish to eat rather than gulping for food at the surface (catfish will eat off the bottom of the tank). Also remember what your fish eat, some fish such as plecos and otto catfish will prefer a sinking algae-based wafer rather than flake.
If you are feeding Coldwater fish such as minnows or danios the same rule as tropical, normally for most fish giving them a small pinch of flakes once a day is enough, just make sure after a few minutes there is no food leftover. If you have indoor goldfish such as fancy goldfish (full bodied with a double tail such as an Oranda, Telescope or Ranchu to name a few breeds), I strongly recommend a sinking pellet, these are normally denser and more nutritious than flake food and because they sink it means the fish will spend less time gulping at the surface. Goldfish are descendant from carp which naturally eat off the bottom of the river, hence why you will see your scaly friend searching along the bottom of the tank from time to time. Defrosted deshelled peas are a great goldfish snack once a week, this helps keep their digestive track moving and healthy.
Good luck with feeding your new fish! Remember if you are ever unsure about how much, how often or what to feed your fish then be sure to reach out to an expert for advice as they will more than likely be happy to help!