FYI, the most teeth in a gear set of a given center distance, and a given pitch occurs when they are straight cut spur gears. As soon as you introduce a helical cut, the number of teeth are reduced. So, this will cause a change in the ratio if your center distance is fixed already.
Helical gears give the ultimate flexibility in choosing the shaft center spacing between the driver and driven, because literally any spacing (with slight variations in ratio) can be accomodated by a change in helix angle and number of teeth. Conversely, square cut spur gearing forces you to use center distances that are a multiple of the gear tooth pitch.
The circular pitch of a straight spur gear is the pitch diameter/number of teeth. This distance is determined by the thickness of the cutter creating the tooth space. So when you angle the cutter across the gear face, you can imagine that you are actually projecting this circular pitch onto the side plane of the gear, thus stretching it. It is a cosine relationship.
Also, there is a certain degree of flexibility in which helix angle you choose. Is this for purposes of a simple noise reduction? From what I've read, 22° is about maximum that makes a noticeable difference in noise reduction.
Helical gears produce a small bit of side thrust. Your gearbox needs to allow for this, by transferring the load properly to the bearings with proper thrust spacers or shoulders.
So you need to describe what, if any flexibility you have in design, because right now, it has only a straight spur gear solution.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)