There is a simple answer to the number of available flavours of Sparky: the ease of usage. The user doesn't have to download the "one and only, proper version" and then fiddle around with it (install a different DE and such). Instead the user downloads the installs the ISO with the DE he wants already preconfigurated. It was made that way to minimize the effort needed by the end user to make the OS usable.
Everything here is being done with a few core things in mind:
1. Simplicity - everything must be as easy as it can (we're regularly working on it).
2. Performance - Sparky is not only meant for new hardware. The old devices should also handle it pretty well (to a degree: some DE are just too much for older hardware, so we've also got something more lightweight).
3. Stability - that's why it's Debian-based.
4. Keepin it current - that's why is based on the Testing repo.
In conclusion: I wouldn't expect the number of flavours to go down in the near future. I would even say that there's a probability that it will go up.