rockm

The problem with adding an additional tree is that the new addition will probably be quite noticeable for years. Since it won't have been subject to the same environment as its companiions, It's bark and other things may not be the same as its companions. Depending upon how you work it into the existing planting, it may appear more loosely rooted and not "part" of the composition. It may not even be the same cultivar of Acer p.--make sure it's a match, or at least close.

Best not to simply plunk it on the periphery, but work the root mass interior to get the new tree inside the composition. That means carving out a place for it from the existing rootball, which, depending on the roots may be easy, or it may be a real problem. You won't really know until you start poking around to see what the root ball looks like.

For what it's worth, if you're adding the new tree just to get to an odd number, it may not be worth it. The Japanese "rule" that plantings have an odd number of trees is a rule that can be easily broken. Even numbered plantings can be just as effective if done well.