Generally, no more than two drains can share a vent. In some circumstances, three or four drains may be connected to the same vent line, but this should only be done with expert guidance.
The following points should be considered before connecting multiple drains to one vent:
• Make sure that the total drainpipe length doesn't exceed 100 feet in total;
• Ensure there are no restrictions on the pipe (such as sharp bends); and
• Check that all branches of pipes lead away from the stack at similar angles.