For 'high phase order power' distribution, multiples of three phases are always used because the phases are derived from ordinary three phase power using various transformer arrangements. You could get other phase counts, but the transformers are more symmetrical if you stick with multiples of three.
In motors, you actually almost always have an _even_ phase count; the most common is _six_ phases. However three of these phases are derived internally as the 'back side' of the three phase supply. If you look at the phase bands in the stator, they are 60 electrical degrees apart, generated by the 'supply' side of phase A, then the 'return' side of phase C, then 'supply' B, then 'return' A, then 'supply' C, then 'return' B, and then back to 'supply' A.
Most of the work done on HPO motors has been with supplies that are multiples of three phases. This is simply because the available transformers, stator lamination stacks, and inverter electronics, are all focused on three phase systems. Usually (though not always) the HPO motor will be arranged so that you have twice as many 'half phases' in the motor as you have 'supply phases'.
HPO machines can be connected 'mesh'; this is the high phase version of the delta connection. You are not limited to connecting the phases in adjacent series; you can make any of a number of symmetric connections; for example in a 7 phase machine the motor coils could be connected between phases 1 and 2, then 2 and 3, then 3 and 4.... or they could be connected 1 and 3, 2 and 4, 3 and 5... or 1 and 4, 2 and 5, 3 and 6....
This particular field is my baby; I've been doing research on high phase order motors for about 10 years, and high phase order mesh connected machines for about 5 years. One nifty aspect of HPO machines is that you can intentionally run harmonics into them in order to _improve_ efficiency, and HPO mesh connected machines respond differently to different harmonics, so you can use harmonics to get some very interesting control capabilities.
I guess that I'm pretty severely tooting my own horn here; I'll edit out the following link if Bill thinks that it is not appropriate, but my company has lots of information on HPO machines on this tutorial page: http://www.chorusmotors.gi/technology/index.shtml
P.S. I'd appreciate it if you could send me a link to the 5 phase wind generators that you mention.