Electrical conduit, whether it be metal (EMT), plastic (PVC), or flexible conduit, each have their own maximum number of electrical wires allowed in the conduit. Their fill capacities are different, even though they may be the same sized conduit.
PVC is a plastic conduit that is often used for underground installations and in wet areas. This conduit comes in ten foot lengths and comes with a coupling formed on one end. PVC can be combined by gluing the male end of one stick of conduit to the female end of another using PVC glue. Since PVC is thicker than metal conduit, it holds fewer wires.
EMT conduit is a metal thin walled conduit that comes in ten foot lengths. These sticks have no couplings on the ends. Couplings and connectors can be added to connect the next piece. These couplings come in setscrew and compression types. Setscrew couplings have two screws, one for each piece of conduit which holds the conduits together and also bonds the ground connection between the conduits.
Flexible conduit (armor-clad) is normally made of twisted metal and is very flexible and can be formed around corners and other obstacles. This conduit is used for devices that may need to be moved around easily. A good example is drop-in fluorescent lighting. Because of its unique ability to bend, shape and move at ease, flexible conduit has a huge advantage over the stiff rigid metal and PVC conduit.
The table below gives you some typical conduit sizes used around the home and the allowable number of electrical wires allowed in each.
Recommended Class for Conductor Fill Calculations