There are no limits to the actual number of or types of fixtures that may be connected to a stack.

Instead of limits to the number or types of fixtures the code has a table listing all the types of fixtures that may be connected to a DWV (drain, waste & vent system) and the table defines the minimum drain & trap size for the fixture and it defines a specific DFU (drainage fixture unit) load value for the fixture.

The code then has another table that lists all the listed pipe sizes and the maximum allowable load on each specific size of pipe for both vertical and horizontal application, expressed in dfu's.

By example, a lavatory bowl is listed as requiring a minimum 1-1/4" drain & trap and it is assigned a load value of 1 dfu.

A WC (water closet -commonly called a toilet) that has a flush of 1.6gallon/flush or less is listed as requiring a 3" drain and it has a load value of 3dfu's.

When designing the DWV system we begin by making a list of all the fixtures in the house and computing the total dfu load.

From that we can check the pipe size chart to determine the size of the house main drain and house sewer. We then lay out the branch lines from the main drain to each respective fixture or fixture group such as bathrooms, kitchens, laundry etc and we compute the dfu load for the branch. Here again, we check the pipe size table to determine the size of branch pipe required. The exception being that if any fixture in the group is required a line larger than the dfu load table requires, we must use the line size required by that fixture. An example,,

A tub/shwer combo is 2dfu's, the lavatory 1dfu and the WC is 3dfu's for a total of 6dfu's. The pipe size table says a 2" pipe may carry up to 6dfu's, however because of the size of solids in the discharge from a WC code requires a WC to have a 3" drain and we cannot reduce the size of a line in the direction of flow so we would be required to make a branch serving a WC with 3" pipe.

Next we have to consider the venting. A length of pipe running from a fixture trap to a vent is defined as a "Fixture arm" while any length of horizontal pipe from a vent downstream to the stack or main drain is defined as a "Vented Branch".

When attaching fixtures to a vertical stack we may not attach any fixture to the stack by means of a fixture arm if it is below a WC being served by the stack. by example, your stack is serving a WC on the second floor so you may not connect a fixture arm directly to the stack on the first floor. The option would then be to install a separate stack for each floor or you could install a vent between the fixture traps on the first floor and the point where the line ties into the stack. In this configuration, the section of pipe from the trap to the vent is a fixture arm, but the section from the vent to the stack is a vented branch, which may be attached to the stack.