I don't know what is the basis of 10 being the magic number, but this enneagram compatibility study confirms some of it. There are quite a few 2-8 and 1-9 pairings:
http://www.9types.com/writeup/enneagram_marriages.html
Yeah, I'm familiar with that data.
And I trust it far more than similarmind's homespun crackpot theory.
There might be a good number of 2-8 and 1-9 pairings, but how about 5-5, 4-6, 7-3?
Hell, the only one of those ten pairings that is actually over-represented is 8 husband, 2 wife.
And the 6 husband, 4 wife pairing is underrepresented, so there's no apparent overall over-representation.
In fact, 1 females are more likely to get with 2s, 5s and 7s, and 1 males with 3s, 5s (2 sd's above chance), and 7s.
Also, the above points to another problem with the similarminds chart: it doesn't differentiate between men and women.
And that 9types data shows that a clear differentiation exists between men and women and the relationships they choose to enter.