Back to basics...each drain needs a vent and each drain needs a trap. You can combine vent lines, but there are rules. You need to install the vent before the drain line (trap arm) out of the trap turns down, otherwise, you'll create an S-trap. The trap arm can only be so long (depends on the pipe diameter) before it must be vented. The 42" is a vertical height ABOVE the floor OR 6" above the flood plane of the thing you are venting, whichever is HIGHER). That number comes from a typical kitchen counter, which is 36" above the floor, then add 6" for good measure. The idea is to keep the vent free of waste - since the waste is governed by gravity, 6" gives a little leeway in case you have a pump on the inlet (say a garbage disposer, dishwasher, washing machine, etc.). You can add as many things to a drain line as you want, as long as the size is adequate and each thing is vented properly. There are practical limits based on the size of the fittings, but there can be many things run into it.