Even though four valves is the commonly accepted norm (two intake and two exhaust), it’s really the engineer’s discretion how they wish to balance the ratio of valves. Two-valve engines are also common but odd numbers like three and five certainly have their place in history as well.
In Yamaha’s case the ratio is three intake valves per two exhaust. Even though there were more- the valves themselves are smaller and lighter than a comparative four valve design. That allows for quicker revs and higher RPM limits. ATVs designed for competition use benefit greatly from being able to “get on the pipe” in a hurry – especially in situations where corners lead to jumps with little run-in.
Yamaha has claimed the design allows for smoother, more linear power delivery. Considering there has been no evidence supporting that additional intake valve requires any additional maintenance or service when compared to the rest of the big bores out there, we wouldn’t spend a whole lot of time worrying about the valve count but rather how the design feels in action.