Hi. I’ve heard rumors that people have had 3 or 4 oxygen tanks for scuba diving. Is this possible if you have the right regulator? Also, if it is, does that mean that it will be increased duration?

Theoretically, you can take as many tanks as you can carry. Recreational divers normally just use a single back-mounted tank for the entire dive.

Technical divers may carry multiple tanks for a number of reasons. A typical configuration for tech dives is to have two tanks mounted on their backs ("doubles"), connected to each other through a specialized valve mechanism called a manifold. The two manifold posts each have a separate regulator mounted to it, and there are isolator valves on each post as well as in between tanks so that the air flow can be cut off at these locations in case of equipment failure. Normally the diver has access to the gas in both tanks through either regulator, so the two tanks are used to increase the total gas supply for the main portion of the dive. The gas used for deep dives is usually something called trimix, which is a mixture of oxygen, nitrogen, and helium and will be used during the deepest portion of the dive.

The tech diver may carry other tanks than just their back-mounted doubles, each with its own separate regulator. They may have one or more tanks filled with "traveling" gas, which they breathe from during the ascent & descent portion of the dive because the bottom mix in their doubles may not contain enough oxygen to maintain consciousness at shallower depths. Finally they may carry one or more "deco" tanks which contain high levels of oxygen (50% to 100%) which are only used at very shallow depths to reduce the amount of time spent decompressing at the end of the dive.

A typical tech diver configuration for a deep dive will be a set of doubles containing trimix (each containing around 100 cubic feet of compressed gas) plus smaller sling-mounted tanks containing a travel gas and a deco gas. They may also bring other tanks along, either to increase the duration of the deep portion of the dive or to use "better" oxygen mixtures at different depths of the dive.