The Lemon Slice nebula is unique, which I've always found curious, since it's far beyond what's reachable. And I'm fairly certain that it's nowhere near a reachable system close enough to be visible. So basically, Frontier gave a unique model to a nebula that can only be seen on the galaxy map.
If memory serves, besides the unique models, there are at least seven nebula models, and roughly the same for planetary nebulae - can't check now, unfortunately. Some are more rare than the others though, especially some from the planetary nebulae.
No idea how difficult it might be to create new nebula models for Elite, but I remember FD saying fairly early on that we're unlikely to get new ones. Although they did introduce a new one for the Pleiades later, but I guess that was central to the alien content. The previous model that they used there wasn't really fitting.