Water heaters come in various sizes and they are usually designed to meet the requirements of a house depending on its size. Of course, you may have a larger family than those who installed the current water heater and have found that the tank does not suit the lifestyle of your household.
The hot water reserve is just being used too quickly and can’t meet the demands of your household, causing there to be no hot water when you need it.
Look up the make and model of your water heater, find out whether it’s an electric water heater or a gas water heater. Try and find the manual or if there are any details about the model written on the information panel to find it’s water capacity.
Is your water heater tankless? If so then the tank capacity would not be an issue in your case as it will be connected to the main water supply and heating it on demand. If not and you think your tank capacity is too low for your needs, you may need to consider upgrading your water heater.
There’s also the option of having a utility hot water heater installed right into your bathroom or another room and is only used by the hot water faucets or shower in there. This will allow your main hot water to be reserved for everything else.