That's an interesting topic for discussion. I expect any paleographer worth their salt would be able to find at least a dozen distinct variations, but it seems to me that these are the obvious ones:
1) The writing of Platt Rogers Spencer, which is the least Spencerian looking to modern eyes.
2) First Compendium style, with rather conservative copperplate-derived capitals.
3) New Compendium style, with some of the lower case eccentricities ironed out, and characterised in particular by the direct oval I and J. This being what most informed people think of, probably.
4) Early business writing, before Palmer put his name to it and placed the emphasis squarely on speed rather than aesthetics. Essentially monoline Spencerian with some simplifications.
5) Ornamental Penmanship, a catch all name for the "dashy" writing, based on Spencerian, which was the preserve of highly competitive professional penmen.
That's without touching on the writing of people not in the trade, but since I'm in the UK my access to and familiarity with that sort of material is very limited. OP in particular comes in many flavours, and there's a huge difference between the restrained approach of someone like Henry Behrensmeyer, for instance, and the flamboyance of Francis Courtney or Louis Madarasz. It's quite unhelpful, actually, because many people seem to hear "Spencerian" and think Madarasz, but that sort of artistic work is really on the fringes and at quite a remove from the practical script developed by the Spencers.
