How many soil stacks should a house have?
The International Residential Code requires only one vent per building.

However, all plumbing internal to the building must be tied into the roof vent. This can be accomplished with PVC (depending on your local codes) running from one vertical stack to another at least 6" above the flood rim of any fixture. In this case the attic works fine.

They cannot be cutoff and capped. Then the fixtures being served would not have a vent.

The requirement for a fixture to be within a certain distance of a vent is a different subject than whether that vent goes all the way through the roof. All vertical vent stacks must be tied together and taken to the main building vent or taken through the roof separately. Sometimes it is easier to run another roof vent than to run all the way across the building to the main vent.

As always check with your local inspector to determine if they are using the IRC or the Uniform Plumbing Code which has different requirements.

Good luck!