The pressure tanks hold considerably less water then their physical size would indicate and are there simply to keep pressure on the water, without one, even a small one, the pump would turn on the instant you cracked open a faucet and turn off the instant you closed it. The membrane inside is mounted towards the bottom of the tank about 1/3 of the way up. It keeps the water and air from physically touching each other helping to maintain the amount of air in the tank. In the old days, there was no membrane and the tank needed a frequent recharge as the air joined with the water. Depending on your needs and budget, the more tank, the better. You can parallel many tanks or get one or more large ones. It's important to be sure the precharge pressure is maintained and should be checked once a year. It's easy to do and can help in the longevity of the tank. As mentioned, the less frequently the pump comes on, the likelyhood the pump will last longer not to mention the contacts that kick the pump in.
