Pressure tanks don't make pressure, pumps make pressure. If you tell us what kind of pump you have, we can tell you how much water and pressure it will build. If you try to use more water than the pump can supply, you won't have any pressure. You are also limited by how much water the well will make. You can't pump 12 GPM if the well will only make 10 GPM. You have to match how much water the well will produce, with a pump that will provide the correct amount of water, at the pressure you need. Then you have to match the amount of water being used, with the amount and pressure the pump can supply.