A little humidity is important to comfort in a home, especially during winter. However, a house with lots of vents can look disordered. If several pipes are going through your roof, you may want to connect them to keep the holes in the roof to a minimum.
Depending on the code, you may be able to combine at least some of those vents. It is also important to verify your local codes to see what's acceptable. If says that a house must have at least one 3-inch-diameter vertical stack, a pipe that starts at the horizontal drain line to the septic or sewer system and exits through the roof. The smaller vents don't need to penetrate the roof, but they have to tie into the stack above the highest fixture in the building.
As an option, if allowed in your area, cut off the secondary vents in the attic at least 6 inches above the ceiling insulation and cap each one with an air-admittance vent. This is a thing with a diaphragm that enables air to enter the stack when water goes down the drain but doesn't allow sewer gases to escape into the house.
Can plumbing vents freeze? In severe cold weather, water vapour in the vent can freeze on the top of the stack, and may close it off completely. Freezing temperature puts your vent at risk. To prevent this from happening, while the weather is good before winter, consult your local plumber about preventive measures to keep your vents safe from extreme cold weather.